are encouraged to replace the former with modulate, which emphasizes the need for an inclusive, predictive paradigm for parasite-host interaction. Evolutionary history and mode of transmission will first be considered separately, then integrated using an equation discussed by Antia et al. (1993) and a model proposed by Lenski and May (1994). Transmission is a spatial factor, defined by host density and specific qualities of host-parasite interaction, which gives direction to the modulation of virulence. Evolution is a temporal factor which determines the extent of the modulation. The selective pressures of the transmission mode act on parasite populations over evolutionary time, favoring an equilibrium level of virulence (Lenski and May 1994). DOES COEVOLUTION DETERMINE VIRULENCE? Incongruent evolution is the colonization of a new host species by a parasite. It is widely reported that such colonizations, when successful, feature high virulence due to the lack of both evolved host defenses and parasitic self-regulation (Esch and Fernandez 1993, Toft et al. 1991). Unsuccessful colonizations must frequently occur when parasites encounter hosts with adequate defenses. In Africa, indigenous ruminants experience low virulence from Trypanosoma brucei infection, while introduced ruminants suffer fatal infections (Esch and Fernandez 1993). There has been no time for the new host to develop immunity, or for the parasite to self-regulate. Virulent colonizations may occur regularly in epizootic-enzootic cycles. Sin Nombre virus, a hemmorhagic fever virus, was epizootic in 1993 after the population of its primary enzootic host, Peromyscus maniculatus, had exploded, increasing the likelihood of transmission to humans (Childs et al. 1995). Sin Nombre exhibited unusually high mortality in human populations (Childs et al. 1995), which were being colonized by the parasite. It is assumed that coevolution of par...
