Louis Pasteur had assumed causality by association: the rotting of wine was associated with an overgrowth of bacteria; the putrefaction of broth was linked to its contact with microorganisms. Koch, in contrast, desired a more formal architecture of causality. First, he had isolated a microorganism from a diseased animal. Next, he’d demonstrated that introducing the pathogen into healthy animals caused the same disease. Then he’d re-isolated the microbe from inoculated animals, grown the organism again in pure form in a culture, and shown that it could re-create the disease.
