Koch’s Postulates (a set of criteria published by Koch in 1890) provide a way to link a pathogen with a disease:
1. A specific causative agent must be observed in every case of a disease.
2. The agent must be isolated from a host displaying the disease and grown in pure culture.
3. When the agent from the pure culture is inoculated into an experimentally healthy, susceptible host, the agent must cause the disease.
4. The agent must be reisolated from the inoculated, diseased experimental host and identified as the original specific causative agent.
When Koch’s Postulates are met, an organism has been proved to be the causative agent of an infectious disease.
It is relatively easy to demonstrate that each postulate is met for bacterial diseases. However, it is more difficult to demonstrate the postulates in viral diseases since viruses cannot be grown in artificial media and must instead be grown in living cells. Furthermore, viruses are usually host specific. Thus, this should be noted—that not all infectious agents will grow in a given media, and the causative agent might not be detected.
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