Based on his research on anthrax and tuberculosis, Koch developed a set of criteria for establishing a causal relationship between a microorganism and a disease. These criteria, known as Koch's postulates, became a cornerstone of medical microbiology.

Koch's postulates state: 1. The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms. 2. The microorganism must be isolated from a diseased organism and grown in pure culture. 3. The cultured microorganism should cause disease when introduced into a healthy organism. 4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

Koch's postulates have served as a fundamental framework for identifying the causative agents of numerous infectious diseases. While some modifications and exceptions have been recognized over time (e.g., for viruses and asymptomatic carriers), the postulates remain a vital principle in medical microbiology and epidemiology.