Genetic variation in the host's response to infections is likely to be present in any host–parasite system. Thus, in most cases, virulence management and the associated changes in disease characteristics affect the genetic composition of the hosts over a time scale comparable to the host's life span. If virulence management is concerned with such time scales and, in particular, if the disease in question has a significant impact on host survival or fertility, host evolution cannot be ignored. In this chapter a modeling framework is developed to allow virulence managers to assess disease-induced host–evolution in a sexually reproducing diploid host, that is, to express fitness in terms of variation in epidemic quantities such as morbidity and infectivity. Thus, we take the consequences of virulence management one step further, and examine the effect of virulence management on the host population. Thereafter, we look further ahead at the consequences of changes in host genetics on pathogen strains.
As a first approximation, variation in disease-induced mortality among hosts causes variation in host survival and hence in fitness. In turn, differential fitness changes the composition of host susceptibilities and thus the prevalence of the disease. This interaction between host composition and disease prevalence is the focus of this chapter; we assume that the immediate consequences of virulence management on prevalence are known and expressed as changes in epidemic parameters.