If the Spanish who arrived to the Americas in the 1400s had become infected with native viruses that felled them, they might never have become the conquistadores.
It was by pure chance that the native Americans were physiologically unprepared for the viruses brought along by the Spanish. That the mere presence of the Spanish would prove deadly for so many natives had the side-effect of re-enforcing the former’s cultural, political and military advantages.
Had the opposite occurred, the Spanish might well have lost faith in their plans, or, at least, a great deal of blood and treasure.
update: oh, there’s a whole body of work about this.