KOCH’S POSTULATES:
1.The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
2.The microorganism must be isolated from a diseased organism and grown in pure culture.
3.The cultured microorganism should cause disease when introduced into a healthy organism.
4.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[1] 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 third postulate specifies "should", not "must", because as Koch himself proved in regard to both tuberculosis and cholera,[2] 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.
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.[3] In summary, a body of evidence that satisfies Koch’s postulates is sufficient but not necessary to establish causation.
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