Virulence is a measure of how destructive the parasite is of its host: it can be measured precisely as the reduction in fitness of a parasitized host relative to an unparasitized host.
A highly virulent parasite is one that kills its host quickly, reducing the hostís fitness to zero. The virulence of a parasite is normally thought to be a side-effect of the manner in which the parasite lives off its host. If, for instance, a parasite consumes a large proportion of its hostís cells, it will be more likely to kill its host and therefore be more virulent than one that consumes less host cells.
When does it benefit a parasite to harm its host?