Robert Koch was a German practicioner. He is also known as the father of microbiology.
According to Koch’s postulates, a microorganism can be accepted as the causative agent of an infectious disease only if the following conditions are fulfilled.
(i) The organism should be constantly associated with the lesions of the disease.
(ii) It should be possible to isolate the organism in pure culture from the lesions of the disease.
(iii) The isolated organism (in pure culture) when inoculated in suitable laboratory animals should produce a similar disease.
(iv) It should be possible to re-isolate the organism in pure culture from the lesions produced in the experimental animals.
Source- textbook of microbiology C P Baveja and Google images