Many experts now believe that the New World was home to 40 million to 50 million people before Columbus arrived and that most of them died within decades. In Mexico alone, the native population fell from roughly 30 million in 1519 to 3 million in 1568. There was similar devastation throughout the Caribbean islands, Central America and Peru. The eminent Yale historian David Brion Davis says this was “the greatest genocide in the history of man.” Yet it’s increasingly clear that most of the carnage had nothing to do with European barbarism. The worst of the suffering was caused not by swords or guns but by germs. By the time Columbus set sail, the people of the Old World held the distinction of being thoroughly diseased. By domesticating pigs, horses, sheep and cattle, they had infected themselves with a wide array of germs. And through centuries of war, exploration, and city-building, they had kept those agents in constant circulation. Virtually any European who crossed the Atlantic during the 16th century had battled such illness as smallpox and measles during childhood and emerged fully immune. By contrast, the people of the Americas had spent thousands of years in biological isolation. By the time Columbus had arrived, groups like the Aztecs and Maya of Central America and Peru’s Incas had built cities large enough to sustain major epidemics. Archeological evidence suggests they suffered from syphilis, tuberculosis, a few intestinal parasites and some types of influenza. Yet they remained untouched by the diseases that had raged for centuries in the Old World. When the newcomers arrived carrying mumps, measles, whooping cough, smallpox, cholera, gonorrhea and yellow fever, the Indians were immunologically defenseless.