Virulent here means "how bad it is to the host species". A virus with high morbidity and mortality, and high contagiousness, is a pretty virulent virus. Now,

Virulent here means "how bad it is to the host species". A virus with high morbidity and mortality, and high contagiousness, is a pretty virulent virus. Now, if it is too virulent, the virus might die off -- become extinct. If it is very contagious but does not sicken or kill the host, then the host won't even notice, and the virus will flourish. If continuing to exist were the virus' goal -it can't exactly have a goal, now can it?-, then being less virulent would be in its best interests. But viruses don't have goals, and this just amounts to an observation: that highly virulent viruses are rare, and that those that survive tend to be the less virulent strains.