asite and host will result in decreased virulence (Esch and Fernandez 1993, Toft et al. 1991). Sin Nombre virus was found to infect 30.4 % of the P. maniculatus population, exhibiting little or no virulence in the mice (Childs et al. 1995). Similar low levels of virulence have been found in the enzootic rodent hosts of Yersinia pestis (Gage et al. 1995). In Australia, decreased grades of virulence of myxoma virus have been observed in rabbit populations since the virus was introduced in 1951 (Krebs C. J. 1994). Many of the most widespread parasites exhibit low virulence, suggesting that success in parasite suprapopulation range and abundance may be the result of reduction in virulence over time. Hookworms are present in the small intestines of one-fifth of the world's human population and rarely induce mortality directly (Hotez 1995). Evolution toward a higher level of virulence has been regarded as an unexplainable anomaly. Parasites which do less harm presumably have an advantage throughout a long coevolutionary association with their hosts. Ebert's (1994) experiment with the planktonic crustacean Daphnia magna and its horizontally transmitted parasite Pleistophora intestinalis suggests that coevolution does not determine the direction of the modulation of virulence. Virulence decreased with the geographic distance between sites of origin where the host and parasite were collected (Ebert 1994). Thus, the parasite was significantly more virulent in hosts it coexisted with in the wild than it was in novel hosts. Many viruses, such as Rabies (Lyssavirus spp.), persist in natural populations while maintaining high levels of virulence in all potential hosts (Krebs, J. W. 1995). Extinction is not an inevitable outcome of increased virulence (Lenski and May 1994). Increased or conserved virulence during coevolution calls into question long held assumptions about the effect of coevolution on parasitic virulenc...
