l. 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 ofYersinia pestis (Gage et al. 1995). In Australia, decreased grades ofvirulence of myxoma virus have been observed in rabbit populations since thevirus was introduced in 1951 (Krebs C. J. 1994). Many of the mostwidespread parasites exhibit low virulence, suggesting that success in parasitesuprapopulation range and abundance may be the result of reduction invirulence over time. Hookworms are present in the small intestines ofone-fifth of the world's human population and rarely induce mortality directly(Hotez 1995). Evolution toward a higher level of virulence has been regardedas an unexplainable anomaly. Parasites which do less harm presumably havean advantage throughout a long coevolutionary association with their hosts.Ebert's (1994) experiment with the planktonic crustacean Daphnia magnaand its horizontally transmitted parasite Pleistophora intestinalis suggests thatcoevolution does not determine the direction of the modulation of virulence.Virulence decreased with the geographic distance between sites of originwhere the host and parasite were collected (Ebert 1994). Thus, the parasitewas significantly more virulent in hosts it coexisted with in the wild than it wasin novel hosts. Many viruses, such as Rabies (Lyssavirus spp.), persist innatural populations while maintaining high levels of virulence in all potentialhosts (Krebs, J. W. 1995). Extinction is not an inevitable outcome ofincreased virulence (Lenski and May 1994). Increased or conservedvirulence during coevolution calls into question long held assumptions aboutthe effect of coevolution on parasitic virulence (Gibbons 1994). Parasitic virulence frequently changes over coevolutionary time, but the length of parasite-host association does not acco...
