1, the same microorganism appears in every case, but does not exist in healthy people;

2. Isolate these microorganisms from the host and obtain pure culture); In a culture medium;

3. Inoculating healthy and sensitive hosts with the pure culture of this microorganism will lead to the recurrence of the same disease;

4. This microorganism can be isolated from the experimental host and cultured again.

If the above four steps are carried out and confirmed, it can be confirmed that the organism is the pathogen of the disease.

Theoretical defects:

In 1893, it was found that some cases of cholera carriers and typhoid Mary had no symptoms, so Koch later deleted the second half of the first principle.

Later, there were similar findings in polio, herpes, AIDS and hepatitis C. Even today, almost all doctors and virologists agree that polio virus can only paralyze a few infected people.

The third principle is not perfect either. Koch himself found in 1884 that cholera, tuberculosis and other diseases may have different manifestations in different individuals. From today's point of view, HIV cannot infect individuals with cc r5δ32 gene deletion.