 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.