Alright I know this sounds utterly ASB, but I'm using this for a TL. I was reading how the cure for small pox (cowpox) could occur naturally in any of the Bovids. Right now some claim to have proven that an American Bison has a virus similar to cowpox, and as such can act as an innoculation. Now Bison at one time lived everywhere from Southern Mexico to Canada to almost the Atlantic ocean.
So for whatever reason when the Spanish make contact and for whatever reason someone links "Contact with sick Bison= Not getting sick." The disease spreads around but instead of 90-95% (?) of the Native American population dying off its more like 30-50%, so smallpox is pretty much handled.
Now going on this (and no arguing with the semi-fact even though it hasn't be proven outside of a single lab) how does the colonization of the New World go?