Koch's postulates ([ˈkɔx]) are four criteria designed to establish a causal relationship between a causative microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884 and refined and published by Koch in 1890. Koch applied the postulates to establish the etiology of anthrax and tuberculosis, but they have been generalized to other diseases.
The postulates 
Koch's postulates are the following:
- 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 culture.
- The cultured microorganism should cause disease when introduced into a healthy organism.
- The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.
However, Koch abandoned the universalist requirement of the first postulate altogether when he discovered asymptomatic carriers of cholera and, later, of typhoid fever. Asymptomatic or subclinical infection carriers are now known to be a common feature of many infectious diseases, especially viruses such as polio, herpes simplex, HIV, and hepatitis C. As a specific example, all doctors and virologists agree that poliovirus causes paralysis in just a few infected subjects, and the success of the polio vaccine in preventing disease supports the conviction that the poliovirus is the causative agent.
The second postulate may also be suspended for certain microorganisms or entities that cannot (at the present time) be grown in pure culture, such as prions responsible for Creutzfeldt–Jakob disease.
The third postulate specifies "should", not "must", because as Koch himself proved in regard to both tuberculosis and cholera, not all organisms exposed to an infectious agent will acquire the infection. Noninfection may be due to such factors as general health and proper immune functioning; acquired immunity from previous exposure or vaccination; or genetic immunity, as with the resistance to malaria conferred by possessing at least one sickle cell allele.
In summary, a body of evidence that satisfies Koch's postulates is sufficient but not necessary to establish causation.
Koch's postulates were developed in the 19th century as general guidelines to identify pathogens that could be isolated with the techniques of the day. Even in Koch's time, it was recognized that some infectious agents were clearly responsible for disease even though they did not fulfill all of the postulates. Attempts to rigidly apply Koch's postulates to the diagnosis of viral diseases in the late 19th century, at a time when viruses could not be seen or isolated in culture, may have impeded the early development of the field of virology. Currently, a number of infectious agents are accepted as the cause of disease despite their not fulfilling all of Koch's postulates. Therefore, while Koch's postulates retain historical importance and continue to inform the approach to microbiologic diagnosis, fulfillment of all four postulates is not required to demonstrate causality.
Koch's postulates have also influenced scientists who examine microbial pathogenesis from a molecular point of view. In the 1980s, a molecular version of Koch's postulates was developed to guide the identification of microbial genes encoding virulence factors.
Koch’s postulates for the 21st century 
Koch’s postulates have played an important role in microbiology, yet they have major limitations. For example, Koch was well aware that in the case of cholera, the causal agent, Vibrio cholerae, could be found in both sick and healthy people, invalidating his first postulate. Furthermore, viral diseases were not yet discovered when Koch formulated his postulates, and there are many viruses that do not cause illness in all infected individuals, a requirement of the first postulate.
More recently, modern nucleic acid-based microbial detection methods have made Koch’s original postulates even less relevant. These nucleic acid-based methods make it possible to identify microbes that are associated with a disease, but in many cases the microbes are uncultivable. Also, nucleic acid-based detection methods are very sensitive, and they can often detect the very low levels of viruses that are found in healthy people without disease.
The use of these new methods has led to revised versions of Koch’s postulates: Fredricks and Relman have suggested the following set of Koch’s postulates for the 21st century:
- A nucleic acid sequence belonging to a putative pathogen should be present in most cases of an infectious disease. Microbial nucleic acids should be found preferentially in those organs or gross anatomic sites known to be diseased, and not in those organs that lack pathology.
- Fewer, or no, copies of pathogen-associated nucleic acid sequences should occur in hosts or tissues without disease.
- With resolution of disease, the copy number of pathogen-associated nucleic acid sequences should decrease or become undetectable. With clinical relapse, the opposite should occur.
- When sequence detection predates disease, or sequence copy number correlates with severity of disease or pathology, the sequence-disease association is more likely to be a causal relationship.
- The nature of the microorganism inferred from the available sequence should be consistent with the known biological characteristics of that group of organisms.
- Tissue-sequence correlates should be sought at the cellular level: efforts should be made to demonstrate specific in situ hybridization of microbial sequence to areas of tissue pathology and to visible microorganisms or to areas where microorganisms are presumed to be located.
- These sequence-based forms of evidence for microbial causation should be reproducible.
See also 
- Koch, R. (1876). "Untersuchungen über Bakterien: V. Die Ätiologie der Milzbrand-Krankheit, begründet auf die Entwicklungsgeschichte des Bacillus anthracis" [Investigations into bacteria: V. The etiology of anthrax, based on the ontogenesis of Bacillus anthracis]. Cohns Beitrage zur Biologie der Pflanzen 2 (2): 277–310.
- Koch Robert (1893). "Über den augenblicklichen Stand der bakteriologischen Choleradiagnose". Zeitschrift für Hygiene und Infectionskrankheiten (in German) 14: 319–333. doi:10.1007/BF02284324.
- Inglis TJ (November 2007). "Principia aetiologica: taking causality beyond Koch's postulates". J. Med. Microbiol. 56 (Pt 11): 1419–22. doi:10.1099/jmm.0.47179-0. PMID 17965339.
- Koch Robert (1884). "2 Die Aetiologie der Tuberkulose". Mitt Kaiser Gesundh. pp. 1–88.
- Walker L, Levine H, Jucker M (2006). "Koch's postulates and infectious proteins.". Acta Neuropathol (Berl) 112 (1): 1–4. doi:10.1007/s00401-006-0072-x. PMID 16703338.
- Koch R (1893). "Ueber den augenblicklichen Stand der bakteriologischen Choleradiagnose". J. Hyg. Inf. 14: 319–33. doi:10.1007/BF02284324.
- Brock TD (1999). Robert Koch: a life in medicine and bacteriology. Washington DC: American Society of Microbiology Press. ISBN 1-55581-143-4.
- Evans AS (May 1976). "Causation and disease: the Henle-Koch postulates revisited". Yale J Biol Med 49 (2): 175–95. PMC 2595276. PMID 782050.
- Jacomo V, Kelly P, Raoult D (2002). "Natural history of Bartonella infections (an exception to Koch's postulate)". Clin Diagn Lab Immunol 9 (1): 8–18. doi:10.1128/CDLI.9.1.8-18.2002. PMC 119901. PMID 11777823.
- Falkow S (1988). "Molecular Koch's postulates applied to microbial pathogenicity". Rev. Infect. Dis. 10 (Suppl 2): S274–6. PMID 3055197.
- Fredericks DN, Relman DA (1996). "Sequence-based identification of microbial pathogens: a reconsideration of Koch's postulates". Clin Microbiol Rev 9 (1): 18–33. PMC 172879. PMID 8665474.