Koch's postulates are a set of "rules" that establish the relationship between a pathogen and a disease. These were developed early in the history of the germ theory of disease.
To fulfill the original postulates:
- The microorganism must be found in all organisms suffering from the disease (the original postulates also said "but not in healthy organisms." Since latency is present in many diseases, this phrase is largely abandoned).
- The microorganism must be isolated from a diseased organism.
- The cultured microorganism should cause disease when introduced into a healthy organism.
- The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.
Modern molecular biology has changed this schema somewhat, and has also allowed for the development of the so-called "molecular Koch's postulates".
These postulates, in their original form, are often used by HIV denialists to try to show that HIV does not cause AIDS, and were used to some extent by the tobacco industry to deny that smoking causes lung cancer.