How does natural selection effect virulence in parasites?

One idea:

Natural selection will usually act to reduce virulence. Parasites depend on their hosts, and if they kill their hosts they will soon be dead too: it has been argued therefore that parasites will evolve to keep their hosts alive.

An objection:

This idea is almost universally rejected by evolutionary biologists. Why? Because it is group selectionist. It is indeed in the long-term interest of a parasite species not to destroy the resource it lives off; but natural selection on individual parasites will favor those that reproduce themselves in the greatest numbers over those that restrain themselves in the interest of preserving their hosts. The short-term individual advantage of greater reproduction will usually outweigh any long-term group or species advantage of reproductive restraint.