History

 It is an article of faith that the enormous loss of Indian life after the Spanish arrived was because the Indians had no immunities to measles, smallpox, and a bunch of other diseases that had been killing people in the Old World long enough that our gene pool had selected the weak ones out. One of the student papers referenced a CDC article that I found fascinating:

Abstract

The native population collapse in 16th century Mexico was a demographic catastrophe with one of the highest death rates in history. Recently developed tree-ring evidence has allowed the levels of precipitation to be reconstructed for north central Mexico, adding to the growing body of epidemiologic evidence and indicating that the 1545 and 1576 epidemics of cocoliztli (Nahuatl for "pest”) were indigenous hemorrhagic fevers transmitted by rodent hosts and aggravated by extreme drought conditions.

Now, this was not entirely without Spanish involvement:

These infections appear to have been aggravated by the extreme climatic conditions of the time and by the poor living conditions and harsh treatment of the native people under the encomienda system of New Spain. The Mexican natives in the encomienda system were treated as virtual slaves, were poorly fed and clothed, and were greatly overworked as farm and mine laborers. This harsh treatment appears to have left them particularly vulnerable to epidemic disease.