jan 1, 1882 - "Koch's Four postulates":
1. The organism must always be present, in every case of the disease.
2. The organism must be isolated from a host containing
the disease and grown in pure culture.
3. Samples of the organism taken from pure culture
must cause the same disease when inoculated into a
healthy, susceptible animal in the laboratory.
4. The organism must be isolated from the inoculated
animal and must be identified as the same original
organism first isolated from the originally diseased host.