Paul W. Ewald (born 1953) is an 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. Education and career Ewald received his BSc in 1975 from the University of California, Irvine, in biological sciences and his PhD in 1980 from the University of Washington, in zoology, with specialization in ecology and evolution. He was formerly a professor of biology at Amherst College, and is currently director of the program in evolutionary medicine in the Biology Department of the University of Louisville. Ideas Ewald asserts, along with a growing body of studies, that many common diseases of unknown origin are likely the result of chronic low-level infections from viruses, bacteria or protozoa. For example, cervical cancer can be caused by the human papilloma virus, some cases of liver cancer are caused by hepatitis C or B and the bacteria Helicobacter pylori has been proven to cause stomach ulcers. Ewald argues that many common diseases of currently unknown etiology, such as cancers, heart attacks, stroke and Alzheimer's, may likewise be also caused by chronic low-level microbial infection. Ewald disagrees with the popular theory that genes alone dictate chronic disease susceptibility. Ewald, whose background is in evolutionary biology, points out that any disease causing gene that reduces survival and reproduction would normally eliminate itself over a number of generations. Ewald says that "chronic diseases, if they are common and damaging, must be powerful eliminators of any genetic instruction that may cause them." One example of this is schizophrenia; patients with this mental illness rarely reproduce. Ewald argues that, just by evolutionary pressures, schizophrenia would have already been eliminated if its causes were strictly genetic; he suggests that in the future, an infectious cause of schizophrenia will be discovered. Ewald explains that purely genetic causes of chronic disease will persist only if a genetic instruction provides a compensating benefit (for example, the disease sickle