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:
The suspected pathogen should be present in all cases of disease but not in a healthy animal
The suspected pathogen should be grown in vitro in pure culture.
Cells from the pure culture of pathogen should cause disease in healthy animals.
The pathogen can be reisolated from the infected animal.
