... After it has been determined that the pathogenic organism is present in the animal body, and after it has been shown that the organism can reproduce in the body and be transmitted from one individual to another, the most important experiment remains to be done....to determine the conditions necessary for growth and reproduction of the microorganism.
Robert KochHowever, on many occasions, I examined normal blood and normal tissues and there was no possibility of overlooking bacteria or confusing them with granular masses of equal size. I never found organisms. Thus, I conclude that bacteria do not occur in healthy human or animal tissues.
Robert KochIf my efforts have led to greater success than usual, this is due, I believe, to the fact that during my wanderings in the field of medicine, I have strayed onto paths where the gold was still lying by the wayside. It takes a little luck to be able to distinguish gold from dross, but that is all.
Robert Koch