A number of other factors may operate:

The number of parasites that infect a host

If the host is infected by one parasite, all the parasitic individuals will be the offspring of the original colonizer and they will all be genetically related brothers and sisters. Kin selection will then operate to reduce any selfish proliferation within the host.

If the host has suffered multiple infections, by contrast, the parasites will be unrelated. Natural selection will favor individual parasites that can consume as much of the host as possible, as fast as possible, before any of the other parasites take advantage of the resource; that is, virulence will increase.

We therefore predict lower virulence in diseases with single than with multiple infections.