It is well-known that native American cultures suffered from disease after Columbus, but it now turns out that native American health had a pre-Columbian decline. From last week's NY Times:

What had not been clearly recognized until now, though, is that the general health of Native Americans had apparently been deteriorating for centuries before 1492.
That is the conclusion of a team of anthropologists, economists and paleopathologists who have completed a wide-ranging study of the health of people living in the Western Hemisphere in the last 7,000 years.
... The researchers used standardized criteria to rate the incidence and degree of these health factors by time and geography. Some trends leapt out from the resulting geography. Some trends leapt out from the resulting index. The healthiest sites for Native Americans were typically the oldest sites, predating Columbus by more than 1,000 years. Then came a marked decline.
"Our research shows that health was on a downward trajectory long before Columbus arrived," Dr. Richard H. Steckel and Dr. Jerome C. Rose, study leaders, wrote in "The Backbone of History: Health and Nutrition in the Western Hemisphere," a book they edited. It was published in August.