Parasitic virulence

Virulence means 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 host’s 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 host’s cells, it will be more likely to kill its host and therefore be more virulent than one that consumes less host cells.