Koch's postulates

Koch's postulates (/ˈkɔːx/) are four criteria designed to establish a causative relationship between a microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884, based on earlier concepts described by Jakob Henle, and refined and published by Koch in 1890. Koch applied the postulates to describe the etiology of cholera and tuberculosis, but they have been controversially generalized to other diseases. These postulates were generated prior to understanding of modern concepts in microbial pathogenesis that cannot be examined using Koch's postulates, including viruses (which are obligate cellular parasites) and asymptomatic carriers. They have largely been supplanted by other criteria such as the Bradford Hill criteria for infectious disease causality in modern public health. Koch's postulates (/ˈkɔːx/) are four criteria designed to establish a causative relationship between a microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884, based on earlier concepts described by Jakob Henle, and refined and published by Koch in 1890. Koch applied the postulates to describe the etiology of cholera and tuberculosis, but they have been controversially generalized to other diseases. These postulates were generated prior to understanding of modern concepts in microbial pathogenesis that cannot be examined using Koch's postulates, including viruses (which are obligate cellular parasites) and asymptomatic carriers. They have largely been supplanted by other criteria such as the Bradford Hill criteria for infectious disease causality in modern public health. Koch's postulates are the following: 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 viral diseases such as polio, herpes simplex, HIV/AIDS, 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. Viruses also require host cells to grow and reproduce and therefore cannot be grown in pure cultures. 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. There are a few other exceptions to Koch's postulates. A single pathogen can cause several disease conditions. Additionally, a single disease condition can be caused by several different microorganisms. Some pathogens cannot be cultured in the lab, and some pathogens only cause disease in humans.