My Dictionary 4

Koch's Postulates 

1. The suspected agent (bacterium, virus, etc.) must be present in every case of the disease.

2. That agent must be isolated and grown in pure culture.

3. The cultured agent must cause the disease when it is inoculated into a healthy, susceptible experimental host.

4. The same agent must be re-isolated from the diseased experimental host. 

Germ Theory of Disease

This states that disease is caused by infections of pathogenic microorganisms (at the time, called germs)