COINFECTION AND THE EVOLUTION OF PARASITE VIRULENCE

Analyses of the selection pressures acting on parasite virulence are m ade more complicated when individual hosts can simultaneously harbour many different strains or genotypes of a parasite. Here we explore the evolutionary dynamics of host-parasite associations in which individu al hosts can be coinfected with many different parasite strains. (We t ake coinfection to mean that each strain transmits at a rate unaffecte d by the presence of others in the same host.) This study thus represe nts the opposite extreme to our earlier work on superinfection in whic h there is a dominance hierarchy such that only the most virulent stra in present in a host is transmitted. For highly diverse populations of parasite strains, we find that such coinfection leads to selection fo r strains whose virulence-levels lie in a relatively narrowband close to the maximum consistent with the parasite's basic preproductive rati o, R(0), exceeding unity.