Europeans kill many Native Americans why?

This question is Biological in nature, but can be simplified to fit the historical context. Basically, if you isolate two populations, they will adapt to their different environments. The environment in Europe differed than that of the Americas; therefore, Europeans adapted in a different manner than American Indians. One of the adaptions stems from the proliferation of various diseases in crowded cities and close proximity to animals (part of the reason the Black Plague was so disastrous). Because of the number of novel diseases in Europe, Europeans had built certain immunities. The immune systems of American Indians, however, were not familiar with the diseases found in Europe. When introduced to the common cold or influenza, the American Indians could not fight them off. Many millions of native peoples in the Americas died as a result.