Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Reduced virulence amongst long-term adapted parasites is sometimes explained as a result of the action of interpopulation, interspecific or species selection. Some authors assume that populations or species of parasites that rapidly liquidate their host population are at a disadvantage compared to populations or species that spread more slowly and thus leave their host population time to regenerate. Consequently, parasites with greater infectiousness are at a disadvantage in interpopulation or interspecies competition.  
This mechanism could actually participate to a certain degree in reducing virulence; on the other hand, as has been mentioned repeatedly, the effectiveness of interpopulation and interspecific selection is generally rather low. If the altruistic behaviour of parasites (for example a reduced rate of multiplication) is maintained only by interpopulation selection, then selfish individuals with greater virulence than that exhibited by the other members of the population can very readily emerge and predominate in the population.
Species selection has a somewhat greater chance of affecting the situation. This could be true in the given case because species of parasites with high virulence endanger the existence of their host population and thus simultaneously increase the probability of their own extinction. In nature, there is a greater chance of encountering parasites exhibiting lower virulence towards their natural hosts, as species with high virulence have probably long been extinct.