Three mechanisms of host resistance to microparasites-avoidance, recovery and tolerance-show different evolutionary dynamics.

Parasite resistant hosts may avoid becoming infected, recover more quickly after infection or survive longer once infected. A model is constructed to examine the evolution of costly host resistance to directly transmitted microparasites and these three distinct mechanisms of avoidance, recovery and tolerance are compared. In each case polymorphism is more likely between very dissimilar strains and resistance (by which we mean the resistant strain alone) is always more likely to occur in hosts with high intrinsic productivity. However, the region where polymorphism occurs is relatively much smaller when resistance is through reduced pathogenicity. In particular, polymorphism with highly resistant strains requires correspondingly high costs. This is in contrast to avoidance or recovery resistance, where polymorphism can also occur when high resistance is associated with small costs due to the inability of highly resistant strains with low susceptibility or high recovery to support the parasite alone and hence resist invasion by the susceptible strain. Relatedly, resistance through avoidance and recovery is favoured in response to less pathogenic parasites. Copyright 1999 Academic Press.