Here's more on the history: http://seercom.com/bluto/smallpox/history.html
Hispanola (modern Haiti and the Dominican Republic) were the first site of European contact. Exposure to smallpox during early Spanish attempts to convert the population into plantation slavery exterminated all 3.5 million inhabitants. They were replaced by African slaves, and this process was repeated throughout the New World for decades. Cortes introduced smallpox in 1520 during his unsuccessful assault on Techotitlan, the Aztec capital, now Mexico City. He had retreated to the coast and was surprised to find no retribution force following. After a few months he sent spies to the capital who reported widespread smallpox, and he returned to conquer the Aztecs in a matter of a few weeks, and established himself as their new god and emperor. The conquest of the continent was very much facilitated by rapid population declines. The history of early settlement was typically one of European settlers moving into an area, fighting aboriginals and over time, finding less and less resistance. The Aztec, Huron, Iroquois, Mohigan and other major nations with populations in the millions were reduced to thousands or eliminated altogether, mostly by smallpox.
