Koch's postulate

Any of four criteria designed to establish a causative relationship between a microbe and a disease: (i) the microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms; (ii) it must be isolated from a diseased organism and grown in pure culture; (iii) the cultured microorganism should cause disease when introduced into a healthy organism; (iv) the microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.