Dr. S.G. Kayes posed a question about virulence helping a parasite invade
new territory. I'm not convinced that virulence itself aids in this, but
I am sure that the conditions that would favor virulence would also favor
infections into previously uninhabited hosts species. The conditions that
favor virulence or allow virulence to develop require a probability of
transmission that is high enough to negate the effects of host damage, as
well as a relatively low degree of relatedness between the infecting
strains. These conditions are most commoly found in overcrowded
situations and can lead to epidemic outbreaks. During such an outbreak,
the large number of parasites will result in a larger than normal
absolute (not proportional) number of novel parasite genotypes as the
result of recombination or mutation. It seems probable that such a
concentration of parasites would create ideal conditions for the invasion
of a new host species. Such an invasion would also be beneficial to the
parasite species because of the increasing loss of susceptible hosts due
to the outbreak. - Derek
