Chapter 4 provides background data about the hunting of apes in central Africa and reviews four different hypotheses concerning the potential mechanism of the original cross-species transmission event from chimpanzee to human. After careful analysis, three of these hypotheses can be rejected: an experimental oral poliomyelitis vaccine in the preparation of which chimpanzee cells were allegedly used, medical experiments during which chimpanzee blood was injected into humans, and testicular transplants pioneered by maverick surgeon Serge Voronoff. The only mechanism that remains plausible is the ‘cut hunter’ theory, namely that a hunter, or perhaps a hunter’s wife, was accidentally infected with the simian virus when injured while handling a chimpanzee carcass in the forest or their village. From estimates that provide an order of magnitude of the frequency of such events, it seems certain that no more than a handful of early twentieth-century hunters were occupationally infected with the chimpanzee virus.