Editing Koch's postulates (section)

==Postulates==
[[File:Koch's Postulates.svg|thumbnail|300px|Koch's postulates of disease.]]
Koch's four postulates are:<ref name=walker2006>{{cite journal|vauthors=Walker L, LeVine H, Jucker M|title=Koch's Postulates and Infectious Proteins|journal=Acta Neuropathologica|year=2006|volume=112|issue=1|pages=1–4|doi=10.1007/s00401-006-0072-x|pmid=16703338|pmc=8544537}}</ref>

# The microorganism must be found in abundance in all organisms suffering from the disease but should not be found in healthy organisms.
# The microorganism must be isolated from a diseased organism and grown in pure [[cell culture|culture]].
# The cultured microorganism should cause disease when introduced into a healthy organism.
# The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.{{efn|The fourth postulate was added later<ref name=walker2006/>}}

However, Koch later abandoned the universalist requirement of the first postulate when he discovered asymptomatic carriers of [[cholera]]<ref name=Koch1893>{{cite journal |doi=10.1007/BF02284324 |title=Ueber den augenblicklichen Stand der bakteriologischen Choleradiagnose |trans-title=About the current state of the bacteriological diagnosis of cholera |language=de |journal=Zeitschrift für Hygiene und Infektionskrankheiten |volume=14 |pages=319–38 |year=1893 | vauthors = Koch R |s2cid=9388121 |url=https://zenodo.org/record/2077681 }}</ref> and, later, of [[typhoid fever]].<ref name="bio.libretexts.org">{{Cite web|date=2018-06-20|title=10.1E: Exceptions to Koch's Postulates|url=https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Microbiology_(Boundless)/10%3A_Epidemiology/10.1%3A_Principles_of_Epidemiology/10.1E%3A_Exceptions_to_Kochs_Postulates|access-date=2021-09-21|website=Biology LibreTexts|language=en}}</ref> [[subclinical infection|Subclinical infections]] and [[asymptomatic carrier]]s are now known to be a common feature of many infectious diseases, especially viral diseases such as [[polio]], [[herpes simplex]], [[HIV/AIDS]], [[hepatitis C]], and [[COVID-19]]. For example, [[poliovirus]] only causes paralysis in a small percentage of those infected.<ref name="bio.libretexts.org"/>

The second postulate does not apply to [[pathogen]]s incapable of growing in pure culture. For example, viruses are dependent on entering and hijacking host cells to use their resources for growth and reproduction, incapable of growing alone.<ref>{{cite journal | vauthors = Inglis TJ | title = Principia aetiologica: taking causality beyond Koch's postulates | journal = Journal of Medical Microbiology | volume = 56 | issue = Pt 11 | pages = 1419–1422 | date = November 2007 | pmid = 17965339 | doi = 10.1099/jmm.0.47179-0 | doi-access = free }}</ref>

The third postulate specifies "should", rather than "must", because Koch's experiments with [[tuberculosis]] and cholera showed that not all organisms exposed to an infectious agent will acquire the infection.<ref name=Koch1884>{{cite journal | vauthors = Koch R |title=Die Aetiologie der Tuberkulose |journal=Mittheilungen aus dem Kaiserlichen Gesundheitsamte (Reports from the Imperial Office of Public Health) |date=1884 |volume=2 |pages=1–88 |url=https://books.google.com/books?id=Y6lQAAAAYAAJ&pg=PA1 |trans-title=The etiology of tuberculosis |language=German}}</ref><!-- is this?: Conferenz zur Erörterung der Cholerafrage im Kaiserl. Gesundheitsamt. Deutsche Medicinische Wochenschrift 10 (1884). S.499-507, 519-532 = Conferenz zur Erörterung
der Cholerafrage 1884. --> Some individuals may avoid infection by maintaining their health for proper immune functioning, acquiring immunity from previous exposure or vaccination, or through genetic immunity, such as [[sickle cell trait]] conferring resistance to [[malaria]].<ref>{{cite journal | vauthors = Archer NM, Petersen N, Clark MA, Buckee CO, Childs LM, Duraisingh MT | title = Resistance to ''Plasmodium falciparum'' in sickle cell trait erythrocytes is driven by oxygen-dependent growth inhibition | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 115 | issue = 28 | pages = 7350–7355 | date = July 2018 | pmid = 29946035 | pmc = 6048551 | doi = 10.1073/pnas.1804388115 | doi-access = free | bibcode = 2018PNAS..115.7350A }}</ref>

Other exceptions to Koch's postulates include evidence that some pathogens can cause several diseases, such as the [[Varicella zoster virus|varicella-zoster virus]] causing [[chickenpox]] and [[shingles]]. Conversely, diseases like [[meningitis]] can be caused by a variety of bacterial, viral, fungal, and parasitic pathogens.<ref>{{Cite book | vauthors = Slonczewski J, Foster J, Zinser E |url=https://digital.wwnorton.com/microbio5 |title=Microbiology: An Evolving Science  |date=June 26, 2020 |publisher=[[W. W. Norton & Company]] |isbn=978-0-393-93447-2 |edition=5th |location=New York, N.Y. |pages=14–20 |url-access=subscription}}</ref>