Many of the deadliest human diseases including those that ravaged the new world like smallpox, influenza, and measles were zoonotic diseases that originated in

Many of the deadliest human diseases including those that ravaged the new world like smallpox, influenza, and measles were zoonotic diseases that originated in domesticated animals. Since the new world didn't have as many domesticated animals, they had fewer zoonotic diseases. It's also largely an accident of history. The Conquistadors inadvertently introduced smallpox suddenly by sending for reinforcements during a Caribbean smallpox outbreak. The Mayans that the Spaniards spent the next two centuries conquering weren't nearly as quickly exposed and died off in fewer numbers from European diseases.