Koch's postulates: In 1890 the German physician and bacteriologist Robert Koch set out his celebrated criteria for judging whether a given bacteria is the cause of a given disease. Koch's criteria brought some much-needed scientific clarity to what was then a very confused field.

Koch's postulates are as follows:

• The bacteria must be present in every case of the disease.
• The bacteria must be isolated from the host with the disease and grown in pure culture.
• The specific disease must be reproduced when a pure culture of the bacteria is inoculated into a healthy susceptible host.
• The bacteria must be recoverable from the experimentally infected host.

However, Koch's postulates have their limitations and so may not always be the last word. They may not hold if:

• The particular bacteria (such as the one that causes leprosy) cannot be "grown in pure culture" in the laboratory.
• There is no animal model of infection with that particular bacteria.

A harmless bacteria may cause disease if:

• It has acquired extra virulence factors making it pathogenic.
• It gains access to deep tissues via trauma, surgery, an IV line, etc.
• It infects an immunocompromised patient.
• Not all people infected by a bacteria may develop disease-subclinical infection is usually more common than clinically obvious infection.

Despite such limitations, Koch's postulates are still a useful benchmark in judging whether there is a cause-and-effect relationship between a bacteria (or any other type of microorganism) and a clinical disease.

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