Koch's postulates

Koch's postulates are four ideas about the relationship between a microbe and a disease. 
The postulates were made by Robert Koch and Friedrich Loeffler in 1884, based on earlier ideas of Jakob Henle. They were refined and published by Koch in 1890. 
Koch applied the postulates to cholera and tuberculosis, but they have been used for other diseases. These postulates were made before viruses were discovered. Also, they did not realise that some people were "asymptomatic carriers": they could carry the disease without showing signs of it. The postulates are little used today. Other criteria are used, for example the Bradford Hill criteria for infectious diseases in modern public health.
Koch's postulates: