Koch's postulates

Koch's postulates (kɒx ) are four criteria designed to establish a causal 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 the statements were refined and published by Koch in 1890. Koch applied the postulates to describe the etiology of cholera and tuberculosis, both of which are now ascribed to bacteria. The postulates have been controversially generalized to other diseases. More modern concepts in microbial pathogenesis cannot be examined using Koch's postulates, including viruses (which are obligate intracellular 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 and the Molecular Koch's postulates for microbial pathogenesis. Koch's four postulates are: 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 re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent. However, Koch later abandoned the universalist requirement of the first postulate when he discovered asymptomatic carriers of cholera and, later, of typhoid fever. Subclinical infections and asymptomatic carriers 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. The second postulate does not apply to pathogens 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.