A well-known result from the theory of the evolution of virulence is the pr
ediction that the virulence of a pathogen (i.e. the rate of parasite-induce
d host mortality) always evolves to higher levels when host background mort
ality rates increase. This prediction, however, is derived from models that
assume that host mortality sources combine additively to determine the ove
rall host mortality rate. In this paper, we suggest that such additivity is
probably rare for many host-pathogen systems, and explore how the predicti
ons for the evolution of virulence are altered when interactions between ho
st mortality sources are incorporated into the theory. Our results indicate
that if mortality-source interactions are sufficiently strong then the evo
lutionarily stable level of virulence can actually decrease as the backgrou
nd mortality rate increases. Consequently, a detailed mechanistic descripti
on of how parasites and other mortality sources combine to cause host morta
lity is required before reliable predictions about virulence evolution can
be made. Moreover, mortality-source interactions make empirical comparisons
of the virulence of different parasites a much more subtle issue.