One of the most important — indeed the only early — sociologist of medical knowledge went unacknowledged in his lifetime and was left in obscurity until a series of chance events in the 1970s led to his rediscovery. We are still sorting through the implications and applications of his work as this chapter shows. Ludwik Fleck spent most of his professional career in medicine, in particular on disorders of the blood. While he gained a solid professional recognition, the anti-Semitism of the pre-war years and the subsequent war cast a long shadow over many Jewish scholars.