Koch's postulates are a set of criteria used to establish a causal relationship between a microorganism and a specific disease. They are as follows:
1. The microorganism must be present in every case of the disease and absent from healthy individuals. 2. The microorganism must be isolated and grown in pure culture. 3. The cultured microorganism should cause the same disease when inoculated into a healthy, susceptible animal. 4. The same microorganism must be re-isolated from the experimentally infected animal.
These postulates were developed by German physician and microbiologist Robert Koch in the late 19th century and have been widely used in the field of microbiology to identify the causative agents of infectious diseases.