Virulence is evolutionarily less constrained in horizontal transmission

A vertically transmitted parasite requires its host to reproduce to provide resources for itself or its immediate offspring; horizontally transmitted hosts have no such requirement.

Consider the success of a more and a less virulent strain of parasite in the two cases. A vertically transmitted parasite experiences a trade-off between making more offspring and the success of those offspring. A parasite that reproduces more will be more virulent as it uses up more of the host; but it will reduce the host’s reproduction and the resources available for its own offspring. This trade-off will place an upper limit on virulence. A horizontally transmitted parasite experiences no such trade-off: the success of its offspring is independent of the reproduction of its host. Virulence is therefore much less constrained.