Professor Jared Diamond explains more of this beautifully in his book Guns, Germs and Steel. Definitely a great read and there is also a Nat Geo documentary narrated by him.
Url changed from <a href="https://medium.com/scientifically-quite-smarter/americapox-the-missing-plague-969ae87e4d0a#.g7wt0pw2v" rel="nofollow">https://medium.com/scientifically-quite-smarter/americapox-t...</a>, which points to this.