Philosophers use historical case studies to support wide-ranging claims about science. This practice is often criticized as problematic. In this paper we suggest that the function of case studies can be understood and justified by analogy to a well-established practice in biology: the investigation of model organisms. We argue that inferences based on case studies are no more (or less) problematic than inferences from model organisms to larger classes of organisms in biology. We demonstrate our view in detail by reference to a case study with a long history: Semmelweis’s discovery of the cause of childbed fever.