New diseases
European explorers unintentionally introduced numerous diseases into the Americas, including smallpox, measles, influenza, typhus, tuberculosis, and diphtheria. These diseases were deadly to Native Americans, who had never been exposed to them and so had never developed immunity to them. Some historians have estimated that European diseases killed up to 95 percent of all people in the New World--tens of millions of inhabitants-- within the first 200 years of the arrival of European explorers.