Why do some parasites kill the host they depend upon while others coexist with their host? Two prime factors determine parasitic virulence: the manner in which the parasite is transmitted, and the evolutionary history of the parasite and its host. Parasites which have colonized a new host species tend to be more virulent than parasites which have coevolved with their hosts. Parasites which are transmitted horizontally tend to be more virulent than those transmitted vertically. It has been assumed that parasite-host interactions inevitably evolve toward lower virulence. This is contradicted by studies in which virulence is conserv