Medicine in Europe and the United States - Koch's Postulates (formalized In 1882)

That the organism could be discoverable in every instance of the disease;

That, extracted from the body, the germ could be produced in a pure culture, maintainable over several microbial generations;

That the disease could be reproduced in experimental animals through a pure culture removed by numerous generations from the organisms initially isolated;

That the organism could be retrieved from the inoculated animal and cultured anew.

SOURCE: Roy Porter, The Greatest Benefit to Mankind: A Medical History of Humanity, (New York and London: Harper Collins, 1997), p. 436.