This article is a few months old, but I haven't seen it covered on this site, and it deserves a mention. From Discover magazine, it postulates that a native disease was actually the most destructive one that the Aztecs faced. This doesn't disprove Diamond's theory, mind you. It's just a reminder that Diamond only claimed that the precolumbian Americas were free of livestock-human and human-human epidemics; he never ruled out pest-human and tropical epidemics.
the Article said:(...) He [Acuña-Soto ] also thinks he may have solved one of the other great mysteries of cocolitzli—namely,
why it hit the Aztecs hard but left the Spanish largely unaffected.
Hemorrhagic viruses affect human populations that are already stressed, Acuña-Soto says. "The natives were poor and probably near starvation and living in unsanitary conditions where the rats would congregate. They also worked in the fields, where they'd be exposed to the rat droppings. The Spanish made up the upper classes."
I suppose that it did not have time to mutate into a less virulent version before running its course. Had it do so then the Aztrecs could have passed it onto their conquerors and wiped them out. In such an AH, the Spanish nobility would have thought twice about going out to Mexico to steal from the natives, sorry, earn a living in administering a land of simple pagans.The virus was usually transmitted from rodents to people, and only from person to person through close contact. The Spanish didn't work in the fields, nor did they have any close contact with any of the Aztecs who did. Therefore, they didn't contract the disease.