The debilitating effect of a parasite, or more typically a genetically-variable population of conspecific parasites, on a host organism. Theoretical and experimental models in biology tend to consider only the elevated mortality of the host when quantifying virulence, although there are other debilitating effects of the parasite(s) that are not mediated through mortality (i.e. through other components of fitness). For example, a high load of parasites may reduce the host's mating success; though debilitating in terms of fitness, this does not necessarily involve host mortality.
Why do parasites harm their hosts? Why, and under what conditions, is virulence the favorable strategy? This has been and is still an active area of research in epidemiology and evolutionary biology, and has produced a large literature of theoretical models (for review, see Frank, S.A. 1996. Models of parasite virulence. Quarterly Review of Biology 71(1): 37-79). Not only does the study of virulence bear significance for our own situation, in which our own parasites are increasingly resistant to drug therapy, but should also reveal important features of the ecology of species interaction, microbiology, and coevolution.