International audience Ludwik Fleck is known mainly for his pioneering studies of science as a social activity. This text investigates a different aspect of Fleck’s epistemological thought—his engagement with normative aspects of medicine and public health and their political underpinnings. In his sinuous professional trajectory, Fleck navigated between two distinct thought styles: fundamental microbiological research and practice-oriented investigations of infectious diseases. Fleck’s awareness of tensions between these two approaches favored the genesis of his theoretical reflections. At the same time, his close observation of medical and epidemiological practices led him to the conclusion that collectively produced scientific facts are situated and fragile. Thought collectives, Fleck explained, can err or yield to external pressures, with potentially disastrous consequences. While Fleck the reflexive experimental scientist has been creatively translated into the science studies idiom, Fleck the reflexive practical microbiologist and public health expert still awaits inspired translation.