Host-Parasite Relations, Vectors, and the Evolution of Disease Severity

A widespread contention in the parasitological and medical literature is that severe disease represents a lack of coadaption between host and parasite2 because parasite species that do not harm their hosts have the best chance of long-term survival. This view has been proposed throughout this century (89, 91, 97) and was extensively developed by Dubos, who concluded: "Given enough time a state of peaceful coexistence eventually becomes established between any host and parasite" (33, p. 190). In other words, commensalism was believed to be the ideal evolutionary end point for both host and parasite. Although several authors have suggested that evolution may sometimes result in increased virulence (6, 7, 14, 27, 28, 61), the former view is often accepted as fact in current texts and. reviews (e.g. 4, 48). One of the major problems with this view, however, is that disease severity is analyzed in terms of the survival of the species or of a large subgroup of the species rather than in terms of the effects on gene frequencies within the species. Evolutionary theory, on the other hand, emphasizes that it is the effects on gene frequencies that yield the evolutionary changes within a species (10 1).