What is Koch’s postulates ?

Koch's postulates are the four criteria established by Robert Koch, the founder of modern bacteriology,  to identify the causative agent of a particular disease.  These four postulates are:
1. The microorganism or other pathogen must be present in all cases of the disease.
2. The pathogen can be isolated from the diseased host and grown in pure culture.
3. The pathogen from the pure culture must cause the disease when inoculated into a healthy susceptible laboratory animal.
4. The pathogen must be re-isolated from the new host and shown to be the same as the originally inoculated pathogen.

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