Koch's postulates

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Koch's postulates (or Henle-Koch postulates) are four criteria that were formulated by Robert Koch in 1884 and refined and published by him in 1890. According to Koch, all four must be fulfilled in order to establish a causal relationship between a parasite and a disease. He applied these to establish the etiology of anthrax and 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.

However, it must be noted that Koch abandoned the second part of the first postulate altogether when he discovered asymptomatic carriers of cholera (Koch, 1893) and later, Typhoid Mary. Indeed, asymptomatic carriers of many diseases have since been found, especially viruses such as polio, herpes simplex, HIV and hepatitis C. As a specific example, all doctors and virologists agree that poliovirus causes paralysis in just a few infected subjects, and the success of the polio vaccine in preventing disease supports the conviction that the poliovirus is the causative agent.

The third postulate does not always happen, as Koch himself discovered and stated in regard to both tuberculosis and cholera (Koch, 1884). Indeed, we see this today with diseases such as HIV, where CCR5 Δ32 deletion individuals seem to be resistant to infection with HIV.

References

Koch R. Über die Ätiologie der Tuberkulose. In: "Verhandlungen des Kongresses für Innere Medizin. Erster Kongress, Wiesbaden 1882".
Koch R. (1884) Mitt Kaiser Gesundh 2, 1-88
Koch R. (1893) J. Hyg. Inf. 14, 319-333

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