time and distance to its probable birthplace in the Himalayan foothills, as were smallpox and measles to their origins in the Fertile Crescent. With the explosion of long-distance commerce during the Roman-Han era, and later under Islamic and Mongol influence, these diseases savaged distant, defenseless populations. Over the ensuing 1,500 years, the once-separated disease pools of the Old World collided and coalesced catastrophically, and in the end largely immunized Asians and Europeans. The first Western migrants to the New World could not even begin to comprehend the devastation they were
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