Koch’s Postulates Help Establish That a Pathogen Causes Disease

Determining whether a particular microbe is actually the cause of a given disease is guided by Koch’s postulates. In the early days of microbiology, when novel bacteria were regularly recovered from diseased persons and animals, Robert Koch proposed a set of rules to help establish causal relationships. His postulates of 1884 are straightforward:

1. The microbe must be detected in all host organisms suffering from the disease.
2. The microbe must be isolated from a diseased host and grown in pure culture.
3. The cultured microbe should cause disease when introduced into a healthy host.
4. The microbe must be isolated from the inoculated, diseased experimental host and shown to be identical to the original microbe suspected of causing the disease.