Editing Infection (section)

===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 infection|subclinical]]'', or [[wikt:occult#Adj1|occult]]. An infection that is inactive or dormant is called a ''latent infection''.<ref>{{cite book |author1=Kayser, Fritz H |author2=Kurt A Bienz |author3=Johannes Eckert |author4=Rolf M Zinkernagel | title = Medical microbiology | publisher = Georg Thieme Verlag | location = Stuttgart | year = 2005 | page = 398 | isbn = 978-3-13-131991-3 }}</ref> An example of a latent bacterial infection is [[latent tuberculosis]]. Some viral infections can also be latent, examples of [[virus latency|latent viral infections]] are any of those from the ''[[Herpesviridae]]'' family.<ref>{{Cite journal|last=Grinde|first=Bjørn|date=2013-10-25|title=Herpesviruses: latency and reactivation – viral strategies and host response|journal=Journal of Oral Microbiology|volume=5|pages=22766|doi=10.3402/jom.v5i0.22766|issn=0901-8328|pmc=3809354|pmid=24167660}}</ref>

The word ''infection'' can [[denotation|denote]] any presence of a particular pathogen at all (no matter how little) but also is often used in a [[word sense|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 professional]]s to speak of ''[[#Colonization|colonization]]'' (rather than ''infection'') when they mean that some of the pathogens are present but that no clinically apparent infection (no disease) is present.<ref>{{Cite journal |last=Dani |first=Arpad |date=2014 |title=Colonization and infection |journal=Central European Journal of Urology |volume=67 |issue=1 |pages=86–87 |doi=10.5173/ceju.2014.01.art19 |issn=2080-4806 |pmc=4074726 |pmid=24982790}}</ref>

====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.<ref>{{Citation|last1=Boldogh|first1=Istvan|title=Persistent Viral Infections|date=1996|url=http://www.ncbi.nlm.nih.gov/books/NBK8538/|work=Medical Microbiology|editor-last=Baron|editor-first=Samuel|edition=4th|publisher=University of Texas Medical Branch at Galveston|isbn=978-0-9631172-1-2|pmid=21413348|access-date=2020-01-23|last2=Albrecht|first2=Thomas|last3=Porter|first3=David D.|archive-date=2020-04-14|archive-url=https://web.archive.org/web/20200414121324/https://www.ncbi.nlm.nih.gov/books/NBK8538/|url-status=live}}</ref> 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 [[bloodborne pathogen|via the bloodstream]] to another area of the body.<ref name=":0">{{Cite book|title= Microbiology|last=Foster|first=John|publisher=Norton|year=2018|isbn=978-0-393-60257-9|location=New York|pages=39}}</ref>

====Primary versus opportunistic====

{{See also|Coinfection}}
{{anchor|Primary versus secondary}}<!-- This section is linked from Primary infection, Secondary infection and Acute infection-->
Among the many varieties of [[microorganisms]], relatively few cause disease in otherwise healthy individuals.<ref name=Baron>This section incorporates [https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed public domain] {{Webarchive|url=https://web.archive.org/web/20090629132331/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed |date=2009-06-29 }} materials included in the text: [https://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=mmed.TOC&depth=10 Medical Microbiology] {{Webarchive|url=https://web.archive.org/web/20090701213330/https://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=mmed.TOC&depth=10 |date=2009-07-01 }} Fourth Edition: [https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.chapter.594 Chapter 8] {{Webarchive|url=https://web.archive.org/web/20090219005050/https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.chapter.594 |date=2009-02-19 }} (1996). Baron, Samuel MD. The University of Texas Medical Branch at Galveston. {{cite book |url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed |title=Medical Microbiology |access-date=2013-11-27 |url-status=live |archive-url=https://web.archive.org/web/20090629132331/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed |archive-date=June 29, 2009 |isbn=9780963117212 |publisher=University of Texas Medical Branch at Galveston |year=1996 |pmid=21413252 |last1=Baron |first1=S. }}</ref> Infectious disease results from the interplay between those few [[pathogen]]s 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 infection|opportunistic pathogens]]''.<ref name="pmid29135922">{{cite journal |vauthors=Patil NK, Guo Y, Luan L, Sherwood ER |title=Targeting Immune Cell Checkpoints during Sepsis |journal=International Journal of Molecular Sciences |volume=18 |issue=11 |date=November 2017 |page=2413 |pmid=29135922 |pmc=5713381 |doi=10.3390/ijms18112413 |url=|doi-access=free }}</ref>

===== 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.<ref name="pmid31027668">{{cite journal |vauthors=McArthur DB |title=Emerging Infectious Diseases |journal=The Nursing Clinics of North America |volume=54 |issue=2 |pages=297–311 |date=June 2019 |pmid=31027668 |pmc=7096727 |doi=10.1016/j.cnur.2019.02.006 |url=}}</ref>

===== Opportunistic pathogens =====

{{main|Opportunistic infection}}
Opportunistic pathogens can cause an infectious disease in a host with depressed resistance ([[immunodeficiency]]) or if they have unusual access to the inside of the body (for example, via [[Major trauma|trauma]]). [[Opportunistic infection]] may be caused by microbes ordinarily in contact with the host, such as [[pathogenic bacteria]] or fungi in the [[gastrointestinal]] or the [[upper respiratory tract]], and they may also result from (otherwise innocuous) microbes acquired from other hosts (as in ''[[Clostridioides difficile]]'' [[colitis]]) or from the environment as a result of [[Physical trauma|traumatic]] introduction (as in [[surgical]] wound infections or [[compound fracture]]s). An opportunistic disease requires impairment of host defenses, which may occur as a result of [[genetic defect]]s (such as [[chronic granulomatous disease]]), exposure to [[antimicrobial]] drugs or [[immunosuppressive]] chemicals (as might occur following [[poison]]ing or [[cancer]] [[chemotherapy]]), exposure to [[ionizing radiation]], or as a result of an infectious disease with immunosuppressive activity (such as with [[measles]], [[malaria]] or [[HIV disease]]). Primary pathogens may also cause more severe disease in a host with depressed resistance than would normally occur in an immunosufficient host.<ref name=Sherris/>

===== Secondary infection =====
While a primary infection can practically be viewed as the [[root cause analysis|root cause]] of an individual's current health problem, a secondary infection is a [[sequela]] or [[complication (medicine)|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).<ref name="pmid29135922"/>

===== Other types of infection =====
Other types of infection consist of mixed, [[Iatrogenic infections|iatrogenic]], [[Hospital-acquired infection|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.<ref name=":0" />

====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]].<ref name="pmid28515626">{{cite journal |vauthors=Barberis I, Bragazzi NL, Galluzzo L, Martini M |title=The history of tuberculosis: from the first historical records to the isolation of Koch's bacillus |journal=Journal of Preventive Medicine and Hygiene |volume=58 |issue=1 |pages=E9–E12 |date=March 2017 |pmid=28515626 |pmc=5432783 |doi= |url=}}</ref>

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 [[microbiological culture|cultured]] ''in 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.<ref name="pmid31440916">{{cite journal |vauthors=Hosainzadegan H, Khalilov R, Gholizadeh P |title=The necessity to revise Koch's postulates and its application to infectious and non-infectious diseases: a mini-review |journal=European Journal of Clinical Microbiology & Infectious Diseases|volume=39 |issue=2 |pages=215–218 |date=February 2020 |pmid=31440916 |doi=10.1007/s10096-019-03681-1 |s2cid=201283277 |url=}}</ref>

[[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 (epidemiology)|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.<ref name="pmid31325286">{{cite journal |vauthors=Riley LW |title=Differentiating Epidemic from Endemic or Sporadic Infectious Disease Occurrence |doi-access=free |s2cid-access=free |journal=Microbiology Spectrum |volume=7 |issue=4 |pages= |date=July 2019 |pmid=31325286 |doi=10.1128/microbiolspec.AME-0007-2019 |s2cid=198135563 |url=|pmc=10957193 }}</ref>

====Contagiousness====
Infectious diseases are sometimes called [[contagious disease]]s 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.<ref>{{cite web|url=https://equine.ca.uky.edu/content/science-sleuths-science-shapes-diagnostic-tests-infectious-or-contagious-%E2%80%93-which-it|title=Science Sleuths: the Science that Shapes Diagnostic Tests: Infectious or Contagious – Which Is It?|publisher=Ag Equine Programs|access-date=2025-01-03}}</ref><ref>{{cite web|url=https://www.britannica.com/dictionary/eb/qa/The-Difference-between-Contagious-and-Infectious-|title=The Difference between 'Contagious' and 'Infectious'|publisher=Britannica}}</ref> 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 [[respiratory droplet|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.<ref>Higurea & Pietrangelo 2016 {{page needed|date=August 2020}}</ref><ref>{{cite web|url=https://globalhealth.harvard.edu/understanding-predictions-what-is-r-naught/|title=Understanding Predictions: What is R-Naught?|publisher=Harvard|date=February 7, 2020}}</ref>

====By anatomic location====
Infections can be classified by the [[Anatomy|anatomic]] location or [[organ system]] infected, including:<ref>{{Cite web |last=Dahal |first=Prashant |date=2023-04-16 |title=Common Human Infections and Causative Agents |url=https://microbenotes.com/human-infections/ |access-date=2025-02-25 |website=microbenotes.com |language=en-US}}</ref>{{citation needed|date=July 2021}}
* [[Urinary tract infection]]
* [[Skin infection]]
* [[Respiratory tract infection]]
* [[Odontogenic infection]] (an infection that originates within a [[tooth]] or in the closely surrounding tissues)
* [[Vaginal infections]]
* [[Intra-amniotic infection]]
In addition, locations of [[inflammation]] where infection is the most common cause include [[pneumonia]], [[meningitis]] and [[salpingitis]].<ref>{{Cite web |last=CDC |date=2024-12-09 |title=Chapter 17: Pneumococcal Disease |url=https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-17-pneumococcal-disease.html |access-date=2025-02-25 |website=Epidemiology and Prevention of Vaccine-Preventable Diseases |language=en-us}}</ref>