Prior to being named to the first chair of Pathology at Geneva University, Friedrich Wilhelm Zahn had published, in 1871, his doctoral thesis entitled Zur Lehre von der Entzundung und Eiterung. This work, written under the direction of Edwin Klebs, at Berne, demonstrated experimentally that the so-called Microsporon septicum (an infectious fungus identified by Klebs) provoked a local inflammation as well as remote secondary focal suppurations. This study apparently confirmed Kleb's hypothesis that Microsporon septicum, usually present in wounds, is the cause of an infectious disease. Zahn's research as well as the work of Klebs fell under attack: the method of identifying the microorganism as well as the demonstration of his causal relationship to inflammation were not conclusive. At the end of the discussion, it was admitted that the status of a cause of infection could be attributed to a microorganism only under the two following conditions: the latter must be identified with Koch's method, and his causal relationship to the infectious disease must be specific. This is precisely what Zahn's thesis was unable to prove. The analysis of his work as well as of the critical arguments enables us to follow and understand the controversy on the cause of inflammation and suppuration at the dawn of the bacteriological age