A new study suggests that an epidemic which killed some 5-8 million people in Mexico was not due to the introduction of smallpox, as has been previously assumed. The real cause might have been the spread of salmonella enterica. As far as I recall, smallpox have probably been introduced by ship-wrecked Spaniards as early as 1511, before the conquerors even formally entered the mainland.
[www.theguardian.com]