Answer:
Koch postulates.
Explanation:
Koch postulates establishes that:
The bacteria must be present in every case of disease.
It must be isolated in culture from the sick host.
When this pure culture is administrated to a healthy host, it will induce the disease.
The bacteria must be isolated in culture from this infected host.
Mycobacterium leprae is an exception to this postulates, since it can not be isolated in pure culture.