Why do parasites kill their hosts? During this past decade, research in three different areas; evolutionary ecology, medical microbiology, and population genetics has provided theory and data that address this and related questions of selection and the evolution and maintenance of parasite virulence. A general theory of parasite–host coevolution and the conditions for selection to favour parasite virulence has been put forth. Considerable advances have been made in elucidating the mechanisms of pathogenicity and inheritance of virulence in bacteria. The population genetic structure and the relation ship between pathogenic and non-pathogenic forms has been determined for a number of species of bacteria. We critically review these developments and their implications for questions of selection and the evolution and maintenance of virulence in bacteria. We postulate how selection may operate on specific types of bacterial virulence and present a general protocol to experimentally test hypotheses concerning selection and the evolution of virulence in bacteria.
