RAther like the Europeans and the bubonic plague during a major portion of the "dark" ages in Europe...
Yes. In the book above, the author lays out a compelling case.
And then smallpox ripped through an already-weakened population that was hungry and thus vulnerable. By the way, they also shaped the land. Indians burning the great plains kept it from becoming forested, and terra preta areas (big islands in the swamp) in the Amazon are native geo-engineering.