The Aztec population of Mexico collapsed from 20 million in 1520 to just 1.6 million a century later, due to epidemics brought by the Spanish to which the indigenous population never developed immunity. In Eurasia, however, such diseases had been present for millennia. Smallpox in particular originated in livestock such as cows, which were not domesticated on the North American continent, nor even at the time of the Indians’ migration to the continent.
