Koch's Postulates

Robert Koch was one of the original researchers into tuberculosis, in the 19th century, and was the first to succeed in isolating the organism from tuberculosis patients.

In an attempt to define what an infectious disease actually is, he formulated his famous postulates, which now bear his name.

Basically, if

1. An organism can be isolated from a host suffering from the disease


2. The organism can be cultured in the laboratory


3. The organism causes the same disease when introduced into another host


4. The organism can be re-isolated from that host


The organism is the cause of the disease and the disease is an infectious disease.

There is an implicit assumption in step three, namely that the other host must have a genetic make-up that causes it to react to the organism in the same way as the original host.

Also, these steps do not apply to all infectious disease. Notably, the bacterium causing leprosy, Mycobacterium leprae, cannot be cultured in the laboratory. However, leprosy is still recognised as an infectious disease.