Koch's Postulates

Koch was one of the original researchers into tuberculosis, in the 19th century. In an attempt to define what an infectious disease actually is, he formulated his famous postulates, which now bears his name.

Basically, if

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

and

2. The organism can be cultured in the laboratory

and

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

and

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

then

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.