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 conserved or increases
over time. A model which encompasses the variability of parasite-host
interactions by synthesizing spatial (transmission) and temporal
(evolutionary) factors is examined. …
