How did Koch prove disease causation?
My method was one of rigorous, repeatable experimentation. I began with clinical observation—examining sick animals or humans. Then, I isolated the suspected microbe from diseased tissue and grew it in a pure culture, free from other organisms. The crucial step involved inoculating a healthy, susceptible animal with this pure culture and observing if it developed the same disease. If it did, I would then recover the very same microbe from the newly infected animal. This four-step process, detailed in works like 'Die Ätiologie der Milzbrand-Krankheit,' provided irrefutable evidence. Let us examine the evidence under the microscope.
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