Passing from Siberia into North America about twelve thousand years ago, the Paleo-Indians spent many generations in the subarctic, where the long and bitter winters discouraged many pathogenic microbes that thrive in warmer climes. Moreover, the arctic rigors tended quickly to kill humans suffering from debilitating diseases, leaving a healthier population of survivors. And as nomadic hunter-gathering peoples scattered over an immense territory, the Paleo-Indians did not sustain the “crowd diseases” that need a steady succession of hosts.
