Jack of Harts

The European Colonization of the Americas was a major mass migration that ironically has its roots in the Mongol Hordes that drove the previous major mass migration. When the Mongols conquered most of the known world and united both the West and the East in one large economic region, they brought Asian germs with them that became the Black Death, a plague that killed somewhere around a third of the European and Middle Eastern populations. Tens of millions of people died, civilization as we knew it collapsed, and only the fact that Northern Europe is a cold and uncomfortable place to live allowed the European survivors to rebuild.

The Europeans who did rebuild sported immune systems capable of fighting way more diseases than their ancestors. They were made of hardier stock, a factor they would use for generations. The Europeans that traveled to the Americas centuries later and brought their germs with them, and the people they named Indians had been separated from their Asian cousins for what most people think is thousands of years. They had no defense for the Black Death or the stew of other diseases brewing in European immune systems.