Koch's postulates

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Koch's postulates are four criteria that Robert Koch published in 1884, which he said must be fulfilled in order to establish a causal relationship between a parasite and a disease. He applied these to establish the etiology of tuberculosis, but they have been generalized to other diseases.

The organism must be found in all animals suffering from the disease, but not in healthy animals.
The organism must be isolated from a diseased animal and grown in pure culture.
The cultured organism should cause disease when introduced into a healthy animal.
The organism must be reisolated from the experimentally infected animal.