Philosophically, ruminations on the human mind and its processes have been around since the times of the ancient Greeks. In 387 BCE, Plato had suggested that the brain was the seat of the mental processes. In 1637, René Descartes posited that humans are born with innate ideas and forwarded the idea of mind-body dualism,
which would come to be known as substance dualism (essentially the idea
that the mind and the body are two separate substances). From that time, major debates ensued through the 19th century regarding whether human thought was solely experiential (empiricism), or included innate knowledge (nativism). Some of those involved in this debate included George Berkeley and John Locke on the side of empiricism, and Immanuel Kant on the side of nativism.
With the philosophical debate continuing, the mid to late 19th
century was a critical time in the development of psychology as a
scientific discipline. Two discoveries that would later play substantial
roles in cognitive psychology were Paul Broca's discovery of the area of the brain largely responsible for language production, and Carl Wernicke's discovery of an area thought to be mostly responsible for comprehension of language. Both areas were subsequently formally named for their founders, and
disruptions of an individual's language production or comprehension due
to trauma or malformation in these areas have come to commonly be known
as Broca's aphasia and Wernicke's aphasia.
From the 1920s to the 1950s, the main approach to psychology was behaviorism.
Initially, its adherents viewed mental events such as thoughts, ideas,
attention, and consciousness as unobservable, hence outside the realm of
a science of psychology. One early pioneer of cognitive psychology,
whose work predated much of behaviorist literature, was Carl Jung. Jung introduced the hypothesis of cognitive functions in his 1921 book Psychological Types. Another pioneer of cognitive psychology, who worked outside the
boundaries (both intellectual and geographical) of behaviorism, was Jean Piaget. From 1926 to the 1950s and into the 1980s, he studied the thoughts, language, and intelligence of children and adults.
In the mid-20th century, four main influences arose that would
inspire and shape cognitive psychology as a formal school of thought:
With the development of new warfare technology during WWII,
the need for a greater understanding of human performance came to
prominence. Problems such as how to best train soldiers to use new
technology and how to deal with matters of attention while under duress
became areas of need for military personnel. Behaviorism provided little if any insight into these matters and it was the work of Donald Broadbent, integrating concepts from human performance research and the recently developed information theory, that forged the way in this area.
Developments in computer science would lead to parallels being drawn
between human thought and the computational functionality of computers,
opening entirely new areas of psychological thought. Allen Newell and Herbert Simon spent years developing the concept of artificial intelligence
(AI) and later worked with cognitive psychologists regarding the
implications of AI. This encouraged a conceptualization of mental
functions patterned on the way that computers handled such things as
memory storage and retrieval, and it opened an important doorway for cognitivism.
Noam Chomsky's 1959 critique of behaviorism, and empiricism more generally, initiated what would come to be known as the "cognitive revolution".
Inside psychology, in criticism of behaviorism, J. S. Bruner, J. J.
Goodnow & G. A. Austin wrote "a study of thinking" in 1956. In 1960,
G. A. Miller, E. Galanter and K. Pribram
wrote their famous "Plans and the Structure of Behavior". The same
year, Bruner and Miller founded the Harvard Center for Cognitive
Studies, which institutionalized the revolution and launched the field
of cognitive science.
Formal recognition of the field involved the establishment of research institutions such as George Mandler's
Center for Human Information Processing in 1964. Mandler described the
origins of cognitive psychology in a 2002 article in the Journal of the
History of the Behavioral Sciences.
Ulric Neisser put the term "cognitive psychology" into common use through his book Cognitive Psychology, published in 1967. Neisser's definition of "cognition" illustrates the then-progressive concept of cognitive processes:
The term "cognition" refers to all processes by which the
sensory input is transformed, reduced, elaborated, stored, recovered,
and used. It is concerned with these processes even when they operate in
the absence of relevant stimulation, as in images and hallucinations.
... Given such a sweeping definition, it is apparent that cognition is
involved in everything a human being might possibly do; that every
psychological phenomenon is a cognitive phenomenon. But although
cognitive psychology is concerned with all human activity rather than
some fraction of it, the concern is from a particular point of view.
Other viewpoints are equally legitimate and necessary. Dynamic psychology,
which begins with motives rather than with sensory input, is a case in
point. Instead of asking how a man's actions and experiences result from
what he saw, remembered, or believed, the dynamic psychologist asks how
they follow from the subject's goals, needs, or instincts.
The main focus of cognitive psychologists is on the mental processes that affect behavior. Those processes include, but are not limited to, the following three stages of memory:
Sensory memory storage: holds sensory information
Short-term memory storage: holds information temporarily for analysis and retrieves information from the Long-term memory.
Long-term memory: holds information over an extended period of time which receives information from the short-term memory.
The psychological definition of attention is "a state of focused awareness on a subset of the available sensation perception information". A key function of attention is to identify irrelevant data and filter
it out, enabling significant data to be distributed to the other mental processes. For example, the human brain may simultaneously receive auditory, visual, olfactory, taste, and tactile
information. The brain is able to consciously handle only a small
subset of this information, and this is accomplished through the
attentional processes.
Attention can be divided into two major attentional systems: exogenous control and endogenous control. Exogenous control works in a bottom-up manner and is responsible for orienting reflex, and pop-out effects. Endogenous control works top-down and is the more deliberate attentional system, responsible for divided attention and conscious processing.
One major focal point relating to attention
within the field of cognitive psychology is the concept of divided
attention. A number of early studies dealt with the ability of a person
wearing headphones to discern meaningful conversation when presented
with different messages into each ear; this is known as the dichotic
listening task. Key findings involved an increased understanding of the mind's ability
to both focus on one message, while still being somewhat aware of
information being taken in from the ear not being consciously attended
to. For example, participants (wearing earphones) may be told that they
will be hearing separate messages in each ear and that they are expected
to attend only to information related to basketball. When the
experiment starts, the message about basketball will be presented to the
left ear and non-relevant information will be presented to the right
ear. At some point the message related to basketball will switch to the
right ear and the non-relevant information to the left ear. When this
happens, the listener is usually able to repeat the entire message at
the end, having attended to the left or right ear only when it was
appropriate. The ability to attend to one conversation in the face of many is known as the cocktail party effect.
Other major findings include that participants cannot comprehend
both passages when shadowing one passage, they cannot report the content
of the unattended message, while they can shadow a message better if
the pitches in each ear are different. However, while deep processing does not occur, early sensory processing
does. Subjects did notice if the pitch of the unattended message
changed or if it ceased altogether, and some even oriented to the
unattended message if their name was mentioned.
Memory
The
two main types of memory are short-term memory and long-term memory;
however, short-term memory has become better understood to be working
memory. Cognitive psychologists often study memory in terms of working memory.
Though working memory is often thought of as just short-term memory,
it is more clearly defined as the ability to process and maintain
temporary information in a wide range of everyday activities in the face
of distraction. The famously known capacity of memory of 7 plus or minus 2 is a combination of both memories in working memory and long-term memory.
One of the classic experiments is by Ebbinghaus, who found the serial position effect where information from the beginning and end of the list of random words were better recalled than those in the center. This primacy and recency effect varies in intensity based on list length. Its typical U-shaped curve can be disrupted by an attention-grabbing word; this is known as the Von Restorff effect.
The Working Memory Model (Baddeley and Hitch, 1974, updated-2000)
Many models of working memory have been made. One of the most regarded is the Baddeley and Hitch model of working memory.
It takes into account both visual and auditory stimuli, long-term
memory to use as a reference, and a central processor to combine and
understand it all.
A large part of memory is forgetting, and there is a large debate among psychologists of decay theory versus interference theory.
Modern conceptions of memory are usually about long-term memory and break it down into three main sub-classes. These three classes are somewhat hierarchical in nature, in terms of the level of conscious thought related to their use.
Procedural memory is memory for the performance of particular types of action. It is often activated on a subconscious level, or at most requires a minimal amount of conscious effort. Procedural memory includes stimulus-response-type information, which is activated through association with particular tasks, routines, etc. A person is using procedural knowledge when they seemingly "automatically" respond in a particular manner to a particular situation or process. An example is driving a car.
Semantic memory is the encyclopedic knowledge that a person possesses. Knowledge like what the Eiffel Tower
looks like, or the name of a friend from sixth grade, represent
semantic memory. Access of semantic memory ranges from slightly to
extremely effortful, depending on a number of variables including but
not limited to recency of encoding of the information, number of
associations it has to other information, frequency of access, and
levels of meaning (how deeply it was processed when it was encoded).
Episodic memory
is the memory of autobiographical events that can be explicitly stated.
It contains all memories that are temporal in nature, such as when one
last brushed one's teeth or where one was when one heard about a major
news event. Episodic memory typically requires the deepest level of conscious thought, as it often pulls together semantic memory and temporal information to formulate the entire memory.
Perception
Perception involves both the physical senses (sight, smell, hearing, taste, touch, and proprioception) as well as the cognitive processes
involved in interpreting those senses. Essentially, it is how people
come to understand the world around them through the interpretation of
stimuli. Early psychologists like Edward B. Titchener began to work with perception in their structuralist approach to psychology. Structuralism
dealt heavily with trying to reduce human thought (or "consciousness",
as Titchener would have called it) into its most basic elements by
gaining an understanding of how an individual perceives particular
stimuli.
Current perspectives on perception within cognitive psychology
tend to focus on particular ways in which the human mind interprets
stimuli from the senses and how these interpretations affect behavior.
An example of the way in which modern psychologists approach the study
of perception is the research being done at the Center for Ecological
Study of Perception and Action at the University of Connecticut (CESPA).
One study at CESPA concerns ways in which individuals perceive their
physical environment and how that influences their navigation through
that environment.
Language
Psychologists have had an interest in the cognitive processes involved with language that dates back to the 1870s, when Carl Wernicke proposed a model for the mental processing of language. Current work on language within the field of cognitive psychology varies widely. Cognitive psychologists may study language acquisition, individual components of language formation (like phonemes), how language use is involved in mood, or numerous other related areas.
Broca's and Wernicke's areas of the brain, which are critical in language
Significant work has focused on understanding the timing of language
acquisition and how it can be used to determine if a child has, or is at
risk of, developing a learning disability.
A study from 2012 showed that, while this can be an effective strategy,
it is important that those making evaluations include all relevant
information when making their assessments. Factors such as individual
variability, socioeconomic status, short-term and long-term memory capacity, and others must be included in order to make valid assessments.
Metacognition
Metacognition,
in a broad sense, is the thoughts that a person has about their own
thoughts. More specifically, metacognition includes things like:
How effective a person is at monitoring their own performance on a given task (self-regulation).
A person's understanding of their capabilities on particular mental tasks.
Much of the current study regarding metacognition within the field of
cognitive psychology deals with its application within the area of
education. Being able to increase a student's metacognitive abilities
has been shown to have a significant impact on their learning and study
habits. One key aspect of this concept is the improvement of students' ability
to set goals and self-regulate effectively to meet those goals. As a
part of this process, it is also important to ensure that students are
realistically evaluating their personal degree of knowledge and setting
realistic goals (another metacognitive task).
Common phenomena related to metacognition include:
Cryptomnesia:
generating thought believing it is unique but it is actually a memory
of a past experience; also known as unconscious plagiarism.
False Fame Effect: non-famous names can be made to be famous.
Validity effect: statements seem more valid upon repeated exposure.
Imagination inflation: imagining an event that did not occur and having increased confidence that it did occur.
Modern perspectives
Modern perspectives on cognitive psychology generally address cognition as a dual process theory, expounded upon by Daniel Kahneman in 2011. Kahneman differentiated the two styles of processing more, calling them
intuition and reasoning. Intuition (or system 1), similar to
associative reasoning, was determined to be fast and automatic, usually
with strong emotional bonds included in the reasoning process. Kahneman
said that this kind of reasoning was based on formed habits and very
difficult to change or manipulate. Reasoning (or system 2) was slower
and much more volatile, being subject to conscious judgments and
attitudes.
Applications
Abnormal psychology
Following
the cognitive revolution, and as a result of many of the principal
discoveries to come out of the field of cognitive psychology, the
discipline of cognitive behavior therapy (CBT) evolved. Aaron T. Beck is generally regarded as the father of cognitive therapy, a particular type of CBT treatment. His work in the areas of recognition and treatment of depression has gained worldwide recognition. In his 1987 book titled Cognitive Therapy of Depression,
Beck puts forth three salient points with regard to his reasoning for
the treatment of depression by means of therapy or therapy and
antidepressants versus using a pharmacological-only approach:
1. Despite the prevalent use of antidepressants, the fact
remains that not all patients respond to them. Beck cites (in 1987)
that only 60 to 65% of patients respond to antidepressants, and recent meta-analyses (a statistical breakdown of multiple studies) show very similar numbers. 2.
Many of those who do respond to antidepressants end up not taking their
medications, for various reasons. They may develop side-effects or have
some form of personal objection to taking the drugs. 3. Beck posits that the use of psychotropic drugs may lead to an eventual breakdown in the individual's coping mechanisms.
His theory is that the person essentially becomes reliant on the
medication as a means of improving mood and fails to practice those
coping techniques typically practiced by healthy individuals to
alleviate the effects of depressive symptoms. By failing to do so, once
the patient is weaned off of the antidepressants, they often are unable
to cope with normal levels of depressed mood and feel driven to
reinstate use of the antidepressants.
Social psychology
Many facets of modern social psychology have roots in research done within the field of cognitive psychology. Social cognition
is a specific sub-set of social psychology that concentrates on
processes that have been of particular focus within cognitive
psychology, specifically applied to human interactions. Gordon B. Moskowitz
defines social cognition as "... the study of the mental processes
involved in perceiving, attending to, remembering, thinking about, and
making sense of the people in our social world".
The development of multiple social information processing
(SIP) models has been influential in studies involving aggressive and
anti-social behavior. Kenneth Dodge's SIP model is one of, if not the
most, empirically supported models relating to aggression. Among his
research, Dodge posits that children who possess a greater ability to
process social information more often display higher levels of socially
acceptable behavior; that the type of social interaction that children
have affects their relationships. His model asserts that there are five steps that an individual proceeds
through when evaluating interactions with other individuals and that
how the person interprets cues is key to their reactionary process.
Developmental psychology
Many
of the prominent names in the field of developmental psychology base
their understanding of development on cognitive models. One of the major
paradigms of developmental psychology, the Theory of Mind
(ToM), deals specifically with the ability of an individual to
effectively understand and attribute cognition to those around them.
This concept typically becomes fully apparent in children between the
ages of 4 and 6. Essentially, before the child develops ToM, they are
unable to understand that those around them can have different thoughts,
ideas, or feelings than themselves. The development of ToM is a matter
of metacognition,
or thinking about one's thoughts. The child must be able to recognize
that they have their own thoughts and in turn, that others possess
thoughts of their own.
One of the foremost minds with regard to developmental
psychology, Jean Piaget, focused much of his attention on cognitive
development from birth through adulthood. Though there have been
considerable challenges to parts of his stages of cognitive development,
they remain a staple in the realm of education. Piaget's concepts and
ideas predated the cognitive revolution but inspired a wealth of
research in the field of cognitive psychology and many of his principles
have been blended with modern theory to synthesize the predominant
views of today.
Educational psychology
Modern
theories of education have applied many concepts that are focal points
of cognitive psychology. Some of the most prominent concepts include:
Metacognition:
Metacognition is a broad concept encompassing all manners of one's
thoughts and knowledge about their own thinking. A key area of
educational focus in this realm is related to self-monitoring, which
relates highly to how well students are able to evaluate their personal
knowledge and apply strategies to improve knowledge in areas in which
they are lacking.
Declarative knowledge and procedural knowledge:
Declarative knowledge is a person's 'encyclopedic' knowledge base,
whereas procedural knowledge is specific knowledge relating to
performing particular tasks. The application of these cognitive
paradigms to education attempts to augment a student's ability to
integrate declarative knowledge into newly learned procedures in an
effort to facilitate accelerated learning.
Knowledge organization:
Applications of cognitive psychology's understanding of how knowledge
is organized in the brain has been a major focus within the field of
education in recent years. The hierarchical method of organizing
information and how that maps well onto the brain's memory are concepts
that have proven extremely beneficial in classrooms.
Personality psychology
Cognitive
therapeutic approaches have received considerable attention in the
treatment of personality disorders in recent years. The approach focuses
on the formation of what it believes to be faulty schemata, centralized
on judgmental biases and general cognitive errors.
Relationship to cognitive science
Cognitive psychology is considered a core aspect of cognitive science,
the interdisciplinary study of mind and mental function, including how
such functions implemented in brains and machines. Cognitive science, as
a unitary field, integrates knowledge, theory and methodology from
psychology, neuroscience, linguistics, philosophy, artificial
intelligence, and anthropology.
It has been argued that cognitive science has been largely
consumed by cognitive psychology, with some scholars even using the
terms interchangeably (see LeMoult & Gotlib for an example). This largely results from early difficulties
integrating the different fields of cognitive science (e.g. psychology
and artificial intelligence), with the resulting divergence of
terminology, methodology and theoretical approach over time rendering
efforts at cohering the disciplines challenging.
Criticisms
Lack of cohesion
Some
observers have suggested that as cognitive psychology became a movement
during the 1970s, the intricacies of the phenomena and processes it
examined meant it also began to lose cohesion as a field of study. In Psychology: Pythagoras to Present,
for example, John Malone writes: "Examinations of late
twentieth-century textbooks dealing with "cognitive psychology", "human
cognition", "cognitive science" and the like quickly reveal that there
are many, many varieties of cognitive psychology and very little
agreement about exactly what may be its domain." This misfortune produced competing models that questioned information-processing approaches to cognitive functioning such as Decision Making and Behavioral Sciences.
Controversies
In the early years of cognitive psychology, behaviorist critics held that the empiricism it pursued was incompatible with the concept of internal mental states. However, cognitive neuroscience
continues to gather evidence of direct correlations between
physiological brain activity and mental states, endorsing the basis for
cognitive psychology.
There is however disagreement between neuropsychologists and cognitive psychologists. Cognitive psychology has produced models of cognition which are not supported by modern brain science. It is often the case that the advocates of different cognitive models form a dialectic
relationship with one another thus affecting empirical research, with
researchers siding with their favorite theory. For example, advocates of
mental model theory have attempted to find evidence that deductive reasoning is based on image thinking, while the advocates of mental logic theory have tried to prove that it is based on verbal thinking, leading to a disorderly picture of the findings from brain imaging and brain lesion
studies. When theoretical claims are put aside, the evidence shows that
interaction depends on the type of task tested, whether of visuospatial
or linguistical orientation; but that there is also an aspect of
reasoning which is not covered by either theory.
Similarly, neurolinguistics has found that it is easier to make sense of brain imaging studies when the theories are left aside. In the field of language cognition research, generative grammar has taken the position that language resides within its private cognitive module, while 'Cognitive Linguistics' goes to the opposite extreme by claiming that language is not an independent function, but operates on general cognitive capacities such as visual processing and motor skills.
Consensus in neuropsychology however takes the middle position that,
while language is a specialized function, it overlaps or interacts with
visual processing. Nonetheless, much of the research in language cognition continues to be
divided along the lines of generative grammar and Cognitive
Linguistics; and this, again, affects adjacent research fields including
language development and language acquisition.
Aninfection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable disease, is an illness resulting from an infection.
Treatment for infections depends on the type of pathogen involved. Common medications include:
Antibiotics for bacterial infections.
Antivirals for viral infections.
Antifungals for fungal infections.
Antiprotozoals for protozoan infections.
Antihelminthics for infections caused by parasitic worms.
Infectious diseases remain a significant global health concern,
causing approximately 9.2 million deaths in 2013 (17% of all deaths). The branch of medicine that focuses on infections is referred to as infectious diseases.
Types
Infections are caused by infectious agents (pathogens) including:
Arthropods such as ticks, mites, fleas, and lice,
can also cause human disease, which conceptually are similar to
infections, but invasion of a human or animal body by these
macroparasites is usually termed infestation.
Signs and symptoms
The signs and symptoms of an infection depend on the type of disease. Some signs of infection affect the whole body generally, such as fatigue, loss of appetite, weight loss, fevers, night sweats, chills, aches and pains. Others are specific to individual body parts, such as skin rashes, coughing, or a runny nose.
In certain cases, infectious diseases may be asymptomatic
for much or even all of their course in a given host. In the latter
case, the disease may only be defined as a "disease" (which by
definition means an illness) in hosts who secondarily become ill after
contact with an asymptomatic carrier. An infection is not synonymous with an infectious disease, as some infections do not cause illness in a host.
Bacterial or viral
As
bacterial and viral infections can both cause the same kinds of
symptoms, it can be difficult to distinguish which is the cause of a
specific infection. Distinguishing the two is important, since viral infections cannot be cured by antibiotics whereas bacterial infections can.
In general, viral infections are systemic. This means they involve
many different parts of the body or more than one body system at the
same time; i.e. a runny nose, sinus congestion, cough, body aches etc.
They can be local at times as in viral conjunctivitis or "pink eye" and herpes. Only a few viral infections are painful, like herpes. The pain of viral infections is often described as itchy or burning.
The classic symptoms of a bacterial infection are localized redness,
heat, swelling and pain. One of the hallmarks of a bacterial infection
is local pain, pain that is in a specific part of the body. For example,
if a cut occurs and is infected with bacteria, pain occurs at the site
of the infection. Bacterial throat pain is often characterized by more
pain on one side of the throat. An ear infection is more likely to be diagnosed as bacterial if the pain occurs in only one ear. A cut that produces pus and milky-colored liquid is most likely infected.
Chain of infection; the chain of events that lead to infection
There is a general chain of events that applies to infections, sometimes called the chain of infection or transmission chain.
The chain of events involves several steps – which include the
infectious agent, reservoir, entering a susceptible host, exit and
transmission to new hosts. Each of the links must be present in a
chronological order for an infection to develop. Understanding these
steps helps health care workers target the infection and prevent it from
occurring in the first place.
Colonization
Infection of an ingrown toenail; there is pus (yellow) and resultant inflammation (redness and swelling around the nail).
Infection begins when an organism successfully enters the body, grows
and multiplies. This is referred to as colonization. Most humans are
not easily infected. Those with compromised or weakened immune systems
have an increased susceptibility to chronic or persistent infections.
Individuals who have a suppressed immune system are particularly susceptible to opportunistic infections. Entrance to the host at host–pathogen interface, generally occurs through the mucosa in orifices like the oral cavity,
nose, eyes, genitalia, anus, or the microbe can enter through open
wounds. While a few organisms can grow at the initial site of entry,
many migrate and cause systemic infection in different organs. Some
pathogens grow within the host cells (intracellular) whereas others grow
freely in bodily fluids.
Wound
colonization refers to non-replicating microorganisms within the wound,
while in infected wounds, replicating organisms exist and tissue is
injured. All multicellular organisms are colonized to some degree by extrinsic organisms, and the vast majority of these exist in either a mutualistic or commensal relationship with the host. An example of the former is the anaerobic bacteria species, which colonizes the mammaliancolon, and an example of the latter are the various species of staphylococcus that exist on human skin.
Neither of these colonizations are considered infections. The
difference between an infection and a colonization is often only a
matter of circumstance. Non-pathogenic organisms can become pathogenic
given specific conditions, and even the most virulent organism requires certain circumstances to cause a compromising infection. Some colonizing bacteria, such as Corynebacteria sp. and Viridans streptococci,
prevent the adhesion and colonization of pathogenic bacteria and thus
have a symbiotic relationship with the host, preventing infection and
speeding wound healing.
This image depicts the steps of pathogenic infection.
The variables involved in the outcome of a host becoming inoculated by a pathogen and the ultimate outcome include:
the route of entry of the pathogen and the access to host regions that it gains
the intrinsic virulence of the particular organism
As an example, several staphylococcal species remain harmless on the skin, but, when present in a normally sterile space, such as in the capsule of a joint or the peritoneum, multiply without resistance and cause harm.
An interesting fact that gas chromatography–mass spectrometry, 16S ribosomal RNA analysis, omics,
and other advanced technologies have made more apparent to humans in
recent decades is that microbial colonization is very common even in
environments that humans think of as being nearly sterile.
Because it is normal to have bacterial colonization, it is difficult to
know which chronic wounds can be classified as infected and how much
risk of progression exists. Despite the huge number of wounds seen in
clinical practice, there are limited quality data for evaluated symptoms
and signs. A review of chronic wounds in the Journal of the American Medical Association's "Rational Clinical Examination Series" quantified the importance of increased pain as an indicator of infection. The review showed that the most useful finding is an increase in the
level of pain [likelihood ratio (LR) range, 11–20] makes infection much
more likely, but the absence of pain (negative likelihood ratio range,
0.64–0.88) does not rule out infection (summary LR 0.64–0.88).
Disease
Disease can arise if the host's protective immune mechanisms are compromised and the organism inflicts damage on the host. Microorganisms can cause tissue damage by releasing a variety of toxins or destructive enzymes. For example, Clostridium tetani releases a toxin that paralyzes muscles, and staphylococcus releases toxins that produce shock and sepsis. Not all infectious agents cause disease in all hosts. For example, less than 5% of individuals infected with polio develop disease. On the other hand, some infectious agents are highly virulent. The prion causing mad cow disease and Creutzfeldt–Jakob disease invariably kills all animals and people that are infected.
Persistent infections occur because the body is unable to clear
the organism after the initial infection. Persistent infections are
characterized by the continual presence of the infectious organism,
often as latent infection with occasional recurrent relapses of active
infection. There are some viruses that can maintain a persistent
infection by infecting different cells of the body. Some viruses once
acquired never leave the body. A typical example is the herpes virus,
which tends to hide in nerves and become reactivated when specific
circumstances arise.
Persistent infections cause millions of deaths globally each year. Chronic infections by parasites account for a high morbidity and mortality in many underdeveloped countries.
For
infecting organisms to survive and repeat the infection cycle in other
hosts, they (or their progeny) must leave an existing reservoir and
cause infection elsewhere. Infection transmission can take place via
many potential routes:
Droplet contact, also known as the respiratory route, and the resultant infection can be termed airborne disease.
If an infected person coughs or sneezes on another person the
microorganisms, suspended in warm, moist droplets, may enter the body
through the nose, mouth or eye surfaces.
Fecal-oral transmission, wherein foodstuffs or water become
contaminated (by people not washing their hands before preparing food,
or untreated sewage being released into a drinking water supply) and the
people who eat and drink them become infected. Common fecal-oral transmitted pathogens include Vibrio cholerae, Giardia species, rotaviruses, Entamoeba histolytica, Escherichia coli, and tape worms. Most of these pathogens cause gastroenteritis.
Oral transmission, diseases that are transmitted primarily by oral means may be caught through direct oral contact such as kissing, or by indirect contact such as by sharing a drinking glass or a cigarette.
Transmission by direct contact, Some diseases that are transmissible by direct contact include athlete's foot, impetigo and warts.
Vehicle transmission, transmission by an inanimate reservoir (food, water, soil).
Vector-borne transmission, transmitted by a vector, which is an organism that does not cause disease itself but that transmits infection by conveying pathogens from one host to another.
The relationship between virulence versus transmissibility is complex; with studies have shown that there were no clear relationship between the two. There is still a small number of evidence that partially suggests a link between virulence and transmissibility.
Diagnosis
Diagnosis of infectious disease sometimes involves identifying an infectious agent either directly or indirectly. In practice most minor infectious diseases such as warts, cutaneousabscesses, respiratory system infections and diarrheal diseases
are diagnosed by their clinical presentation and treated without
knowledge of the specific causative agent. Conclusions about the cause
of the disease are based upon the likelihood that a patient came in
contact with a particular agent, the presence of a microbe in a
community, and other epidemiological considerations. Given sufficient
effort, all known infectious agents can be specifically identified.
Diagnosis of infectious disease is nearly always initiated by medical history
and physical examination. More detailed identification techniques
involve the culture of infectious agents isolated from a patient.
Culture allows identification of infectious organisms by examining their
microscopic features, by detecting the presence of substances produced
by pathogens, and by directly identifying an organism by its genotype.
Many infectious organisms are identified without culture and
microscopy. This is especially true for viruses, which cannot grow in
culture. For some suspected pathogens, doctors may conduct tests that
examine a patient's blood or other body fluids for antigens or antibodies that indicate presence of a specific pathogen that the doctor suspects.
Other techniques (such as X-rays, CAT scans, PET scans or NMR)
are used to produce images of internal abnormalities resulting from the
growth of an infectious agent. The images are useful in detection of,
for example, a bone abscess or a spongiform encephalopathy produced by a prion.
The benefits of identification, however, are often greatly
outweighed by the cost, as often there is no specific treatment, the
cause is obvious, or the outcome of an infection is likely to be benign.
Symptomatic diagnostics
The
diagnosis is aided by the presenting symptoms in any individual with an
infectious disease, yet it usually needs additional diagnostic
techniques to confirm the suspicion. Some signs are specifically
characteristic and indicative of a disease and are called pathognomonic signs; but these are rare. Not all infections are symptomatic.
In children the presence of cyanosis, rapid breathing, poor peripheral perfusion, or a petechial rash increases the risk of a serious infection by greater than 5 fold. Other important indicators include parental concern, clinical instinct, and temperature greater than 40 °C.
Many diagnostic approaches depend on microbiological culture to isolate a pathogen from the appropriate clinical specimen. In a microbial culture, a growth medium
is provided for a specific agent. A sample taken from potentially
diseased tissue or fluid is then tested for the presence of an
infectious agent able to grow within that medium. Many pathogenic bacteria are easily grown on nutrient agar,
a form of solid medium that supplies carbohydrates and proteins
necessary for growth, along with copious amounts of water. A single
bacterium will grow into a visible mound on the surface of the plate
called a colony,
which may be separated from other colonies or melded together into a
"lawn". The size, color, shape and form of a colony is characteristic of
the bacterial species, its specific genetic makeup (its strain),
and the environment that supports its growth. Other ingredients are
often added to the plate to aid in identification. Plates may contain
substances that permit the growth of some bacteria and not others, or
that change color in response to certain bacteria and not others.
Bacteriological plates such as these are commonly used in the clinical
identification of infectious bacterium. Microbial culture may also be
used in the identification of viruses:
the medium, in this case, being cells grown in culture that the virus
can infect, and then alter or kill. In the case of viral identification,
a region of dead cells results from viral growth, and is called a
"plaque". Eukaryoticparasites may also be grown in culture as a means of identifying a particular agent.
In the absence of suitable plate culture techniques, some microbes require culture within live animals. Bacteria such as Mycobacterium leprae and Treponema pallidum
can be grown in animals, although serological and microscopic
techniques make the use of live animals unnecessary. Viruses are also
usually identified using alternatives to growth in culture or animals.
Some viruses may be grown in embryonated
eggs. Another useful identification method is Xenodiagnosis, or the use
of a vector to support the growth of an infectious agent. Chagas disease is the most significant example, because it is difficult to directly demonstrate the presence of the causative agent, Trypanosoma cruzi in a patient, which therefore makes it difficult to definitively make a diagnosis. In this case, xenodiagnosis involves the use of the vector of the Chagas agent T. cruzi, an uninfected triatomine bug, which takes a blood meal from a person suspected of having been infected. The bug is later inspected for growth of T. cruzi within its gut.
Microscopy
Another principal tool in the diagnosis of infectious disease is microscopy. Virtually all of the culture techniques discussed above rely, at some
point, on microscopic examination for definitive identification of the
infectious agent. Microscopy may be carried out with simple instruments,
such as the compound light microscope, or with instruments as complex as an electron microscope.
Samples obtained from patients may be viewed directly under the light
microscope, and can often rapidly lead to identification. Microscopy is
often also used in conjunction with biochemicalstaining techniques, and can be made exquisitely specific when used in combination with antibody based techniques. For example, the use of antibodies made artificially fluorescent (fluorescently labeled antibodies) can be directed to bind to and identify a specific antigens present on a pathogen. A fluorescence microscope
is then used to detect fluorescently labeled antibodies bound to
internalized antigens within clinical samples or cultured cells. This
technique is especially useful in the diagnosis of viral diseases, where
the light microscope is incapable of identifying a virus directly.
Other microscopic procedures may also aid in identifying
infectious agents. Almost all cells readily stain with a number of basic
dyes due to the electrostatic
attraction between negatively charged cellular molecules and the
positive charge on the dye. A cell is normally transparent under a
microscope, and using a stain increases the contrast of a cell with its
background. Staining a cell with a dye such as Giemsa stain or crystal violet
allows a microscopist to describe its size, shape, internal and
external components and its associations with other cells. The response
of bacteria to different staining procedures is used in the taxonomic classification of microbes as well. Two methods, the Gram stain and the acid-fast
stain, are the standard approaches used to classify bacteria and to
diagnosis of disease. The Gram stain identifies the bacterial groups Bacillota and Actinomycetota, both of which contain many significant human pathogens. The acid-fast staining procedure identifies the Actinomycetota genera Mycobacterium and Nocardia.
Biochemical tests
Biochemical tests used in the identification of infectious agents include the detection of metabolic or enzymatic products characteristic of a particular infectious agent. Since bacteria ferment carbohydrates in patterns characteristic of their genus and species, the detection of fermentation products is commonly used in bacterial identification. Acids, alcohols and gases are usually detected in these tests when bacteria are grown in selective liquid or solid media.
The isolation of enzymes
from infected tissue can also provide the basis of a biochemical
diagnosis of an infectious disease. For example, humans can make neither
RNA replicases nor reverse transcriptase,
and the presence of these enzymes are characteristic., of specific
types of viral infections. The ability of the viral protein hemagglutinin to bind red blood cells
together into a detectable matrix may also be characterized as a
biochemical test for viral infection, although strictly speaking
hemagglutinin is not an enzyme and has no metabolic function.
Serological
methods are highly sensitive, specific and often extremely rapid tests
used to identify microorganisms. These tests are based upon the ability
of an antibody to bind specifically to an antigen. The antigen, usually a
protein or carbohydrate made by an infectious agent, is bound by the
antibody. This binding then sets off a chain of events that can be
visibly obvious in various ways, dependent upon the test. For example, "Strep throat" is often diagnosed within minutes, and is based on the appearance of antigens made by the causative agent, S. pyogenes,
that is retrieved from a patient's throat with a cotton swab.
Serological tests, if available, are usually the preferred route of
identification, however the tests are costly to develop and the reagents
used in the test often require refrigeration.
Some serological methods are extremely costly, although when commonly
used, such as with the "strep test", they can be inexpensive.
Complex serological techniques have been developed into what are known as immunoassays.
Immunoassays can use the basic antibody – antigen binding as the basis
to produce an electro-magnetic or particle radiation signal, which can
be detected by some form of instrumentation. Signal of unknowns can be
compared to that of standards allowing quantitation of the target
antigen. To aid in the diagnosis of infectious diseases, immunoassays
can detect or measure antigens from either infectious agents or proteins
generated by an infected organism in response to a foreign agent. For
example, immunoassay A may detect the presence of a surface protein from
a virus particle. Immunoassay B on the other hand may detect or measure
antibodies produced by an organism's immune system that are made to
neutralize and allow the destruction of the virus.
Instrumentation can be used to read extremely small signals
created by secondary reactions linked to the antibody – antigen binding.
Instrumentation can control sampling, reagent use, reaction times,
signal detection, calculation of results, and data management to yield a
cost-effective automated process for diagnosis of infectious disease.
PCR-based diagnostics
Nucleic acid testing conducted using an Abbott Laboratories ID Now device
Technologies based upon the polymerase chain reaction
(PCR) method will become nearly ubiquitous gold standards of
diagnostics of the near future, for several reasons. First, the catalog
of infectious agents has grown to the point that virtually all of the
significant infectious agents of the human population have been
identified. Second, an infectious agent must grow within the human body
to cause disease; essentially it must amplify its own nucleic acids to
cause a disease. This amplification of nucleic acid in infected tissue
offers an opportunity to detect the infectious agent by using PCR.
Third, the essential tools for directing PCR, primers, are derived from the genomes of infectious agents, and with time those genomes will be known if they are not already.
Thus, the technological ability to detect any infectious agent
rapidly and specifically is currently available. The only remaining
blockades to the use of PCR as a standard tool of diagnosis are in its
cost and application, neither of which is insurmountable. The diagnosis
of a few diseases will not benefit from the development of PCR methods,
such as some of the clostridial diseases (tetanus and botulism).
These diseases are fundamentally biological poisonings by relatively
small numbers of infectious bacteria that produce extremely potent neurotoxins.
A significant proliferation of the infectious agent does not occur,
this limits the ability of PCR to detect the presence of any bacteria.
Metagenomic sequencing
Given the wide range of bacterial, viral, fungal, protozoal, and
helminthic pathogens that cause debilitating and life-threatening
illnesses, the ability to quickly identify the cause of infection is
important yet often challenging. For example, more than half of cases of
encephalitis, a severe illness affecting the brain, remain undiagnosed, despite extensive testing using the standard of care (microbiological culture) and state-of-the-art clinical laboratory methods. Metagenomic
sequencing-based diagnostic tests are currently being developed for
clinical use and show promise as a sensitive, specific, and rapid way to
diagnose infection using a single all-encompassing test. This test is similar to current PCR tests; however, an untargeted whole genome amplification is used rather than primers for a specific infectious agent. This amplification step is followed by next-generation sequencing or third-generation sequencing, alignment comparisons, and taxonomic classification using large databases of thousands of pathogen and commensal reference genomes. Simultaneously, antimicrobial resistance genes within pathogen and plasmid
genomes are sequenced and aligned to the taxonomically classified
pathogen genomes to generate an antimicrobial resistance profile –
analogous to antibiotic sensitivity testing – to facilitate antimicrobial stewardship and allow for the optimization of treatment using the most effective drugs for a patient's infection.
Metagenomic sequencing could prove especially useful for diagnosis when the patient is immunocompromised.
An ever-wider array of infectious agents can cause serious harm to
individuals with immunosuppression, so clinical screening must often be
broader. Additionally, the expression of symptoms is often atypical,
making a clinical diagnosis based on presentation more difficult.
Thirdly, diagnostic methods that rely on the detection of antibodies are
more likely to fail. A rapid, sensitive, specific, and untargeted test
for all known human pathogens that detects the presence of the
organism's DNA rather than antibodies is therefore highly desirable.
Indication of tests
A temporary drive-in testing site for COVID-19 set up with tents in a parking lot
There is usually an indication
for a specific identification of an infectious agent only when such
identification can aid in the treatment or prevention of the disease, or
to advance knowledge of the course of an illness prior to the
development of effective therapeutic or preventative measures. For
example, in the early 1980s, prior to the appearance of AZT for the treatment of AIDS,
the course of the disease was closely followed by monitoring the
composition of patient blood samples, even though the outcome would not
offer the patient any further treatment options. In part, these studies
on the appearance of HIV in specific communities permitted the advancement of hypotheses
as to the route of transmission of the virus. By understanding how the
disease was transmitted, resources could be targeted to the communities
at greatest risk in campaigns aimed at reducing the number of new
infections. The specific serological diagnostic identification, and later genotypic or molecular identification, of HIV also enabled the development of hypotheses as to the temporal and geographical origins of the virus, as well as a myriad of other hypothesis. The development of molecular diagnostic tools have enabled physicians
and researchers to monitor the efficacy of treatment with anti-retroviral drugs.
Molecular diagnostics are now commonly used to identify HIV in healthy
people long before the onset of illness and have been used to
demonstrate the existence of people who are genetically resistant to HIV
infection. Thus, while there still is no cure for AIDS, there is great
therapeutic and predictive benefit to identifying the virus and
monitoring the virus levels within the blood of infected individuals,
both for the patient and for the community at large.
Classification
Subclinical versus clinical (latent versus apparent)
Symptomatic infections are apparent and clinical, whereas an infection that is active but does not produce noticeable symptoms may be called inapparent,silent,subclinical, or occult. An infection that is inactive or dormant is called a latent infection. An example of a latent bacterial infection is latent tuberculosis. Some viral infections can also be latent, examples of latent viral infections are any of those from the Herpesviridae family.
The word infection can denote any presence of a particular pathogen at all (no matter how little) but also is often used in a sense implying a clinically apparent infection (in other words, a case of infectious disease). This fact occasionally creates some ambiguity or prompts some usage discussion; to get around this it is common for health professionals to speak of colonization (rather than infection) when they mean that some of the pathogens are present but that no clinically apparent infection (no disease) is present.
Course of infection
Different terms are used to describe how and where infections present over time. In an acute infection, symptoms develop rapidly; its course can either be rapid or protracted. In chronic infection, symptoms usually develop gradually over weeks or months and are slow to resolve. In subacute
infections, symptoms take longer to develop than in acute infections
but arise more quickly than those of chronic infections. A focal infection is an initial site of infection from which organisms travel via the bloodstream to another area of the body.
Among the many varieties of microorganisms, relatively few cause disease in otherwise healthy individuals. Infectious disease results from the interplay between those few pathogens
and the defenses of the hosts they infect. The appearance and severity
of disease resulting from any pathogen depend upon the ability of that
pathogen to damage the host as well as the ability of the host to resist
the pathogen. However, a host's immune system can also cause damage to
the host itself in an attempt to control the infection. Clinicians,
therefore, classify infectious microorganisms or microbes according to
the status of host defenses – either as primary pathogens or as opportunistic pathogens.
Primary pathogens
Primary pathogens cause disease as a result of their presence or activity within the normal, healthy host, and their intrinsic virulence
(the severity of the disease they cause) is, in part, a necessary
consequence of their need to reproduce and spread. Many of the most
common primary pathogens of humans only infect humans, however, many
serious diseases are caused by organisms acquired from the environment
or that infect non-human hosts.
While a primary infection can practically be viewed as the root cause of an individual's current health problem, a secondary infection is a sequela or complication of that root cause. For example, an infection due to a burn or penetrating trauma
(the root cause) is a secondary infection. Primary pathogens often
cause primary infection and often cause secondary infection. Usually,
opportunistic infections are viewed as secondary infections (because
immunodeficiency or injury was the predisposing factor).
Other types of infection
Other types of infection consist of mixed, iatrogenic, nosocomial,
and community-acquired infection. A mixed infection is an infection
that is caused by two or more pathogens. An example of this is appendicitis, which is caused by Bacteroides fragilis and Escherichia coli.
The second is an iatrogenic infection. This type of infection is one
that is transmitted from a health care worker to a patient. A nosocomial
infection is also one that occurs in a health care setting. Nosocomial
infections are those that are acquired during a hospital stay. Lastly, a
community-acquired infection is one in which the infection is acquired
from a whole community.
Infectious or not
One manner of proving that a given disease is infectious, is to satisfy Koch's postulates (first proposed by Robert Koch), which require that first, the infectious agent
be identifiable only in patients who have the disease, and not in
healthy controls, and second, that patients who contract the infectious
agent also develop the disease. These postulates were first used in the
discovery that Mycobacteria species cause tuberculosis.
However, Koch's postulates cannot usually be tested in modern
practice for ethical reasons. Proving them would require experimental
infection of a healthy individual with a pathogen
produced as a pure culture. Conversely, even clearly infectious
diseases do not always meet the infectious criteria; for example, Treponema pallidum, the causative spirochete of syphilis, cannot be culturedin vitro – however the organism can be cultured in rabbit testes.
It is less clear that a pure culture comes from an animal source
serving as host than it is when derived from microbes derived from plate
culture.
Epidemiology,
or the study and analysis of who, why and where disease occurs, and
what determines whether various populations have a disease, is another
important tool used to understand infectious disease. Epidemiologists
may determine differences among groups within a population, such as
whether certain age groups have a greater or lesser rate of infection;
whether groups living in different neighborhoods are more likely to be
infected; and by other factors, such as gender and race. Researchers
also may assess whether a disease outbreak is sporadic, or just an occasional occurrence; endemic, with a steady level of regular cases occurring in a region; epidemic, with a fast arising, and unusually high number of cases in a region; or pandemic,
which is a global epidemic. If the cause of the infectious disease is
unknown, epidemiology can be used to assist with tracking down the
sources of infection.
Contagiousness
Infectious diseases are sometimes called contagious diseases when they are easily transmitted by contact with an ill person or their secretions (e.g., influenza).
Thus, a contagious disease is a subset of infectious disease that is
especially infective or easily transmitted. All contagious diseases are
infectious, but not vice versa.Other types of infectious, transmissible, or communicable diseases with
more specialized routes of infection, such as vector transmission or
sexual transmission, are usually not regarded as "contagious", and often
do not require medical isolation (sometimes loosely called quarantine)
of those affected. However, this specialized connotation of the word
"contagious" and "contagious disease" (easy transmissibility) is not
always respected in popular use.
Infectious diseases are commonly transmitted from person to
person through direct contact. The types of direct contact are through
person to person and droplet spread.
Indirect contact such as airborne transmission, contaminated objects,
food and drinking water, animal person contact, animal reservoirs,
insect bites, and environmental reservoirs are another way infectious
diseases are transmitted. The basic reproduction number of an infectious disease measures how easily it spreads through direct or indirect contact.
By anatomic location
Infections can be classified by the anatomic location or organ system infected, including:
Washing one's hands, a form of hygiene, is an effective way to prevent the spread of infectious disease.
Techniques like hand washing, wearing gowns, and wearing face masks
can help prevent infections from being passed from one person to
another. Aseptic technique
was introduced in medicine and surgery in the late 19th century and
greatly reduced the incidence of infections caused by surgery. Frequent hand washing remains the most important defense against the spread of unwanted organisms. There are other forms of prevention such as avoiding the use of illicit drugs, using a condom,
wearing gloves, and having a healthy lifestyle with a balanced diet and
regular exercise. Cooking foods well and avoiding foods that have been
left outside for a long time is also important.
disinfectants, which destroy microorganisms found on non-living objects.
antibiotics, called prophylactic
when given as prevention rather as treatment of infection. However,
long term use of antibiotics leads to resistance of bacteria. While
humans do not become immune to antibiotics, the bacteria does. Thus,
avoiding using antibiotics longer than necessary helps preventing
bacteria from forming mutations that aide in antibiotic resistance.
One of the ways to prevent or slow down the transmission of
infectious diseases is to recognize the different characteristics of
various diseases. Some critical disease characteristics that should be evaluated include virulence, distance traveled by those affected, and level of contagiousness. The human strains of Ebola
virus, for example, incapacitate those infected extremely quickly and
kill them soon after. As a result, those affected by this disease do not
have the opportunity to travel very far from the initial infection
zone. Also, this virus must spread through skin lesions or permeable membranes such as the eye. Thus, the initial stage of Ebola
is not very contagious since its victims experience only internal
hemorrhaging. As a result of the above features, the spread of Ebola is
very rapid and usually stays within a relatively confined geographical
area. In contrast, the human immunodeficiency virus (HIV) kills its victims very slowly by attacking their immune system. As a result, many of its victims transmit the virus to other
individuals before even realizing that they are carrying the disease.
Also, the relatively low virulence allows its victims to travel long
distances, increasing the likelihood of an epidemic.
Another effective way to decrease the transmission rate of infectious diseases is to recognize the effects of small-world networks. In epidemics, there are often extensive interactions within hubs or
groups of infected individuals and other interactions within discrete
hubs of susceptible individuals. Despite the low interaction between
discrete hubs, the disease can jump and spread in a susceptible hub via a
single or few interactions with an infected hub. Thus, infection rates
in small-world networks can be reduced somewhat if interactions between
individuals within infected hubs are eliminated (Figure 1). However,
infection rates can be drastically reduced if the main focus is on the
prevention of transmission jumps between hubs. The use of needle
exchange programs in areas with a high density of drug users with HIV is
an example of the successful implementation of this treatment method. Another example is the use of ring culling or vaccination of
potentially susceptible livestock in adjacent farms to prevent the
spread of the foot-and-mouth virus in 2001.
A general method to prevent transmission of vector-borne pathogens is pest control.
In cases where infection is merely suspected, individuals may be quarantined
until the incubation period has passed and the disease manifests itself
or the person remains healthy. Groups may undergo quarantine, or in the
case of communities, a cordon sanitaire may be imposed to prevent infection from spreading beyond the community, or in the case of protective sequestration, into a community. Public health authorities may implement other forms of social distancing, such as school closings, lockdowns or temporary restrictions (e.g. circuit breakers) to control an epidemic.
Immunity
Mary Mallon (a.k.a. Typhoid Mary) was an asymptomatic carrier of typhoid fever. Over the course of her career as a cook, she infected 53 people, three of whom died.
Infection with most pathogens does not result in death of the host
and the offending organism is ultimately cleared after the symptoms of
the disease have waned. This process requires immune mechanisms to kill or inactivate the inoculum of the pathogen. Specific acquired immunity against infectious diseases may be mediated by antibodies and/or T lymphocytes. Immunity mediated by these two factors may be manifested by:
a direct effect upon a pathogen, such as antibody-initiated complement-dependent bacteriolysis, opsonoization, phagocytosis and killing, as occurs for some bacteria,
neutralization of viruses so that these organisms cannot enter cells,
or by T lymphocytes, which will kill a cell parasitized by a microorganism.
The immune system response to a microorganism often causes symptoms such as a high fever and inflammation, and has the potential to be more devastating than direct damage caused by a microbe.
Resistance to infection (immunity) may be acquired following a disease, by asymptomatic carriage of the pathogen, by harboring an organism with a similar structure (crossreacting), or by vaccination.
Knowledge of the protective antigens and specific acquired host immune
factors is more complete for primary pathogens than for opportunistic pathogens. There is also the phenomenon of herd immunity
which offers a measure of protection to those otherwise vulnerable
people when a large enough proportion of the population has acquired
immunity from certain infections.
Immune resistance to an infectious disease requires a critical
level of either antigen-specific antibodies and/or T cells when the host
encounters the pathogen. Some individuals develop natural serum antibodies to the surface polysaccharides
of some agents although they have had little or no contact with the
agent, these natural antibodies confer specific protection to adults and
are passively transmitted to newborns.
Host genetic factors
The
organism that is the target of an infecting action of a specific
infectious agent is called the host. The host harbouring an agent that
is in a mature or sexually active stage phase is called the definitive
host. The intermediate host comes in contact during the larvae stage. A
host can be anything living and can attain to asexual and sexual
reproduction. The clearance of the pathogens, either treatment-induced or spontaneous,
it can be influenced by the genetic variants carried by the individual
patients. For instance, for genotype 1 hepatitis C treated with Pegylated interferon-alpha-2a or Pegylated interferon-alpha-2b (brand names Pegasys or PEG-Intron) combined with ribavirin,
it has been shown that genetic polymorphisms near the human IL28B gene,
encoding interferon lambda 3, are associated with significant
differences in the treatment-induced clearance of the virus. This
finding, originally reported in Nature, showed that genotype 1 hepatitis C patients carrying certain genetic
variant alleles near the IL28B gene are more possibly to achieve
sustained virological response after the treatment than others. Later
report from Nature demonstrated that the same genetic variants are also associated with the natural clearance of the genotype 1 hepatitis C virus.
Treatments
When infection attacks the body, anti-infective drugs
can suppress the infection. Several broad types of anti-infective drugs
exist, depending on the type of organism targeted; they include
antibacterial (antibiotic; including antitubercular), antiviral, antifungal and antiparasitic (including antiprotozoal and antihelminthic)
agents. Depending on the severity and the type of infection, the
antibiotic may be given by mouth or by injection, or may be applied topically. Severe infections of the brain are usually treated with intravenous antibiotics. Sometimes, multiple antibiotics are used in case there is resistance
to one antibiotic. Antibiotics only work for bacteria and do not affect
viruses. Antibiotics work by slowing down the multiplication of
bacteria or killing the bacteria. The most common classes of antibiotics
used in medicine include penicillin, cephalosporins, aminoglycosides, macrolides, quinolones and tetracyclines.
Not all infections require treatment, and for many self-limiting infections the treatment may cause more side-effects than benefits. Antimicrobial stewardship
is the concept that healthcare providers should treat an infection with
an antimicrobial that specifically works well for the target pathogen
for the shortest amount of time and to only treat when there is a known
or highly suspected pathogen that will respond to the medication.
Susceptibility to infection
Pandemics such as COVID-19
show that people dramatically differ in their susceptibility to
infection. This may be because of general health, age, or their immune
status, e.g. when they have been infected previously. However, it also
has become clear that there are genetic factor which determine
susceptibility to infection. For instance, up to 40% of SARS-CoV-2 infections may be asymptomatic, suggesting that many people are naturally protected from disease. Large genetic studies have defined risk factors for severe SARS-CoV-2 infections, and genome sequences
from 659 patients with severe COVID-19 revealed genetic variants that
appear to be associated with life-threatening disease. One gene
identified in these studies is type I interferon
(IFN). Autoantibodies against type I IFNs were found in up to 13.7% of
patients with life-threatening COVID-19, indicating that a complex
interaction between genetics and the immune system is important for natural resistance to Covid.
Similarly, mutations in the ERAP2 gene, encoding endoplasmic reticulum aminopeptidase 2, seem to increase the susceptibility to the plague, the disease caused by an infection with the bacteria Yersinia pestis.
People who inherited two copies of a complete variant of the gene were
twice as likely to have survived the plague as those who inherited two
copies of a truncated variant.
Susceptibility also determined the epidemiology of infection,
given that different populations have different genetic and
environmental conditions that affect infections.
Note: Other causes of death include maternal and perinatal conditions (5.2%), nutritional deficiencies (0.9%), noncommunicable conditions (58.8%), and injuries (9.1%).
The top three single agent/disease killers are HIV/AIDS, TB and malaria.
While the number of deaths due to nearly every disease have decreased,
deaths due to HIV/AIDS have increased fourfold. Childhood diseases
include pertussis, poliomyelitis, diphtheria, measles and tetanus.
Children also make up a large percentage of lower respiratory and
diarrheal deaths. In 2012, approximately 3.1 million people have died
due to lower respiratory infections, making it the number 4 leading
cause of death in the world.
The Great Plague of Marseille in 1720 killed 100,000 people in the city and the surrounding provinces.
With their potential for unpredictable and explosive impacts, infectious diseases have been major actors in human history. A pandemic (or global epidemic) is a disease that affects people over an extensive geographical area. For example:
Plague of Justinian, from 541 to 542, killed between 50% and 60% of Europe's population.
The Black Death
of 1347 to 1352 killed 25 million in Europe over five years. The plague
reduced the old world population from an estimated 450 million to
between 350 and 375 million in the 14th century.
The introduction of smallpox, measles, and typhus
to the areas of Central and South America by European explorers during
the 15th and 16th centuries caused pandemics among the native
inhabitants. Between 1518 and 1568 disease pandemics are said to have
caused the population of Mexico to fall from 20 million to 3 million.
The first European influenza epidemic occurred between 1556 and 1560, with an estimated mortality rate of 20%.
Smallpox killed an estimated 60 million Europeans during the 18th century (approximately 400,000 per year). Up to 30% of those infected, including 80% of the children under 5
years of age, died from the disease, and one-third of the survivors went
blind.
In the 19th century, tuberculosis killed an estimated one-quarter of the adult population of Europe; by 1918 one in six deaths in France were still caused by TB.
The Influenza Pandemic of 1918 (or the Spanish flu) killed 25–50 million people (about 2% of world population of 1.7 billion). Today Influenza kills about 250,000 to 500,000 worldwide each year.
In 2021, COVID-19 emerged as a major global health crisis,
directly causing 8.7 million deaths, making it one of the leading causes
of mortality worldwide.
Emerging diseases
In most cases, microorganisms live in harmony with their hosts via mutual or commensal interactions. Diseases can emerge when existing parasites become pathogenic or when new pathogenic parasites enter a new host.
Human activity is involved with many emerging infectious diseases, such as environmental change enabling a parasite to occupy new niches. When that happens, a pathogen that had been confined to a remote habitat has a wider distribution and possibly a new host organism. Parasites jumping from nonhuman to human hosts are known as zoonoses. Under disease invasion, when a parasite invades a new host species, it may become pathogenic in the new host.
Encroachment on wildlife habitats.
The construction of new villages and housing developments in rural
areas force animals to live in dense populations, creating opportunities
for microbes to mutate and emerge.
Changes in agriculture.
The introduction of new crops attracts new crop pests and the microbes
they carry to farming communities, exposing people to unfamiliar
diseases.
The destruction of rain forests.
As countries make use of their rain forests, by building roads through
forests and clearing areas for settlement or commercial ventures, people
encounter insects and other animals harboring previously unknown
microorganisms.
Uncontrolled urbanization.
The rapid growth of cities in many developing countries tends to
concentrate large numbers of people into crowded areas with poor
sanitation. These conditions foster transmission of contagious diseases.
Modern transport.
Ships and other cargo carriers often harbor unintended "passengers",
that can spread diseases to faraway destinations. While with
international jet-airplane travel, people infected with a disease can
carry it to distant lands, or home to their families, before their first
symptoms appear.
In Antiquity, the Greek historian Thucydides (c. 460 – c. 400 BCE) was the first person to write, in his account of the plague of Athens, that diseases could spread from an infected person to others. In his On the Different Types of Fever (c. 175 AD), the Greco-Roman physician Galen speculated that plagues were spread by "certain seeds of plague", which were present in the air. In the Sushruta Samhita, the ancient Indian physician Sushruta
theorized: "Leprosy, fever, consumption, diseases of the eye, and other
infectious diseases spread from one person to another by sexual union,
physical contact, eating together, sleeping together, sitting together,
and the use of same clothes, garlands and pastes." This book has been dated to about the sixth century BC.
A basic form of contagion theory was proposed by Persian physician Ibn Sina (known as Avicenna in Europe) in The Canon of Medicine (1025), which later became the most authoritative medical textbook in Europe up until the 16th century. In Book IV of the Canon, Ibn Sina discussed epidemics, outlining the classical miasma theory
and attempting to blend it with his own early contagion theory. He
mentioned that people can transmit disease to others by breath, noted
contagion with tuberculosis, and discussed the transmission of disease through water and dirt. The concept of invisible contagion was later discussed by several Islamic scholars in the Ayyubid Sultanate who referred to them as najasat ("impure substances"). The fiqh scholar Ibn al-Haj al-Abdari (c. 1250–1336), while discussing Islamic diet and hygiene,
gave warnings about how contagion can contaminate water, food, and
garments, and could spread through the water supply, and may have
implied contagion to be unseen particles.
When the Black Deathbubonic plague reached Al-Andalus in the 14th century, the Arab physicians Ibn Khatima (c. 1369) and Ibn al-Khatib
(1313–1374) hypothesised that infectious diseases were caused by
"minute bodies" and described how they can be transmitted through
garments, vessels and earrings. Ideas of contagion became more popular in Europe during the Renaissance, particularly through the writing of the Italian physician Girolamo Fracastoro. Anton van Leeuwenhoek (1632–1723) advanced the science of microscopy by being the first to observe microorganisms, allowing for easy visualization of bacteria.
In the mid-19th century John Snow and William Budd
did important work demonstrating the contagiousness of typhoid and
cholera through contaminated water. Both are credited with decreasing
epidemics of cholera in their towns by implementing measures to prevent
contamination of water. Louis Pasteur proved beyond doubt that certain diseases are caused by infectious agents, and developed a vaccine for rabies. Robert Koch provided the study of infectious diseases with a scientific basis known as Koch's postulates. Edward Jenner, Jonas Salk and Albert Sabin developed effective vaccines for smallpox and polio, which would later result in the eradication and near-eradication of these diseases, respectively. Alexander Fleming discovered the world's first antibiotic, penicillin, which Florey and Chain then developed. Gerhard Domagk developed sulphonamides, the first broad spectrum synthetic antibacterial drugs.
The disease has not responded to first line antibiotics;
The disease might be dangerous to other patients, and the patient might have to be isolated
Society and culture
Several
studies have reported associations between pathogen load in an area and
human behavior. Higher pathogen load is associated with decreased size
of ethnic and religious groups in an area. This may be due high pathogen
load favoring avoidance of other groups, which may reduce pathogen
transmission, or a high pathogen load preventing the creation of large
settlements and armies that enforce a common culture. Higher pathogen
load is also associated with more restricted sexual behavior, which may
reduce pathogen transmission. It also associated with higher preferences
for health and attractiveness in mates. Higher fertility rates
and shorter or less parental care per child is another association that
may be a compensation for the higher mortality rate. There is also an
association with polygyny
which may be due to higher pathogen load, making selecting males with a
high genetic resistance increasingly important. Higher pathogen load is
also associated with more collectivism and less individualism, which
may limit contacts with outside groups and infections. There are
alternative explanations for at least some of the associations although
some of these explanations may in turn ultimately be due to pathogen
load. Thus, polygyny may also be due to a lower male: female ratio in
these areas but this may ultimately be due to male infants having
increased mortality from infectious diseases. Another example is that
poor socioeconomic factors may ultimately in part be due to high
pathogen load preventing economic development.
Evidence of infection in fossil remains is a subject of interest for paleopathologists,
scientists who study occurrences of injuries and illness in extinct
life forms. Signs of infection have been discovered in the bones of
carnivorous dinosaurs. When present, however, these infections seem to
tend to be confined to only small regions of the body. A skull
attributed to the early carnivorous dinosaur Herrerasaurus ischigualastensis
exhibits pit-like wounds surrounded by swollen and porous bone. The
unusual texture of the bone around the wounds suggests they were
affected by a short-lived, non-lethal infection. Scientists who studied
the skull speculated that the bite marks were received in a fight with
another Herrerasaurus. Other carnivorous dinosaurs with documented evidence of infection include Acrocanthosaurus, Allosaurus, Tyrannosaurus and a tyrannosaur from the Kirtland Formation. The infections from both tyrannosaurs were received by being bitten during a fight, like the Herrerasaurus specimen.
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