The beginning of the Columbian Exchange...

Whereas in March 1520, when the Spanish fleet arrived, Mexico was home to 22 million people, by December only 14 million were still alive.  Smallpox was only the first blow.  While the new Spanish masters were busy enriching themselves and exploiting the natives, deadly waves of flu, measles and other infectious diseases struck Mexico one after the other, until in 1580 its population was down to less than 2 million.

-Yuval Noah Harari,  Homo Deus:  A Brief History of Tomorrow