Robert Koch's set of postulates judging whether a disease was caused by a specified bacterium. The criteria are:

    The bacterium must be present in every case of the disease.

    The bacterium must be cultured from a diseased host.

    When the cultured bacterium is introduced to a healthy host, it must reproduce the disease.

    The bacterium must be obtainable from the experimental host.

Koch's postulates are useful as a quick rule of thumb, but falls short in several ways. First, normally healthy bacteria can become pathogenic in certain situations: access to other tissues, compromised immune systems, etc. Second, subclinical infection happens far more than clearly symptomatic infections, so mere innoculation may not reveal the relationship between a disease and a bacterium. Third, for any serious disease, animal hosts are a necessity. The disease may not be infectious for that specific species. Fourth, the criteria only apply to bacteria. Both viruses and prions, while perfectly capable of causing disease, are not culturable in a lab dish. Even some bacteria are impossible to grow in such a way.

The larger importance of the postulates was the clearcut understanding it provided of the germ theory of disease for a fledging field. Koch developed the rules during his experiments on the tuberculosis bacterium, proving the presence of the tubercule bacillus, the culturing of the bacterium and the reproduction of the disease through innoculation of guinea pigs. He published the results as The Etiology of Tuberculosis in 1884, and was awarded the Nobel prize in Medicine and Physiology in 1905 for his discovery.

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