This ground-breaking work is the first book to present a Darwinian perspective on infectious disease. It views disease-producing bacteria and viruses as parasites and explains the history of disease as a host-parasite relationship, one which can evolve in many different ways and with radically different effects on the host population. The author's evolutionary approach is interdisciplinary, drawing on theory and example from the fields of epidemiology, molecular genetics, biochemistry, physiology, evolutionary ecology, and the ecology of populations and communities.