Paul W. Ewald (born c. 1953) is an American evolutionary biologist, specializing in the evolutionary ecology of parasitism, evolutionary medicine, agonistic behavior, and pollination biology. He is the author of Evolution of Infectious Disease (1994) and Plague Time: The New Germ Theory of Disease (2002), and is currently director of the program in Evolutionary Medicine at the Biology Department of the University of Louisville. Ewald is known for his "theory of virulence”, suggesting that "the deadlier the germ, the less likely it is to spread", and his theory that many common diseases of unknown origin are likely the result of chronic low-level infections from viruses, bacteria or protozoa. Source: Wikipedia (en)