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.

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

Reference

• Koch R. Über die Ätiologie der Tuberkulose. In: "Verhandlungen des Kongresses für Innere Medizin. Erster Kongress, Wiesbaden 1882".