“From a modern microbiological viewpoint the motivation behind the experiments that led to the discovery of bacteriophage is hard to understand. What was the purpose of filtering a bacterial culture to remove the bacteria, then remixing the filtrate with a fresh bacterial culture? We all know the outcome of this classic experiment, but why was it done in the first place? To fathom the intent of the original investigator, Félix d’Herelle, one has to put aside twenty-first century ideas and recover the context of microbiology and disease in the early twentieth century.“