[WalterBright]

> it was exceedingly deadly to non co-evolved hosts indicates it was not the disease that became less deadly, but that the co-evolved hosts developed better defenses

That's a good argument, but it is not proof that there wasn't some adaptation of smallpox to Europeans. The immune systems of Europeans and Indians diverged 10,000 years ago.

> Rabies had and continues to have a nearly 100% fatality rate in co-evolved hosts for thousands of years.

I doubt that there were large enough epidemics of rabies to influence its evolution.

> Peaceful coexistence is more likely a artifact of the specific dynamic than any sort of meaningful fundamental advantage to reduced deadliness.

Killing your host does not help propagation of the disease. Causing your host to cough and sneeze is a great way to propagate.