Minutus

Smallpox, I believe. They (Europeans) had immunity due to centuries of exposure. The Native Americans did not.

Scientists have also recently found that Native American societies in our Southwest were exterminated by diseases (Hanta virus maybe?) caused by the weather, basically, because years of drought were followed by a very rainy year and small animals suddenly flourished and with them--fleas bearing something, a virus or whatever that devastated tribes. They realized this due to studying tree rings... Sorry this is not more precise; I found it fascinating when I read it in NG. Teresa ( Northern VA)

Thanks Teresa, it looks like you're right--and looks like I was actually thinking of the Incas, not the Mayas. This snippet from Wikipedia:

"Disease decimating the population is commonly listed as the reason for this decline in population. This happened with the Inca Empire, defeated by Francisco Pizarro in 1531. The first epidemic of smallpox was recorded in 1529 and killed the emperor Huayna Capac , the father of Atahualpa , as well as a large portion of the population. New epidemics of smallpox broke out in 1533, 1535, 1558 and 1565, as well as typhus in 1546, influenza in 1558, diphtheria in 1614 and measles in 1618.[10] Dobyns estimated that 90% of the population of the Inca Empire died in these epidemics.[ 11 ] "
"Beware of strangers bearing gifts..." SIgh.
Shannon