Koch's Postulates by Wikipedia

What are Koch's postulates designed to establish?
Koch's postulates are designed to establish a causal relationship between a microbe and a disease.

How many criteria are there in Koch's postulates?
There are four criteria in Koch's postulates.

What are Koch's four postulates?
Koch's four postulates are:

  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 to a healthy organism.
  4. The microogranism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.