The arrival of Europeans had an immense impact on Native American populations, introducing horses, guns and diseases that spread across the continent ahead of European settlements. Native Americans lacked the natural or acquired immunity to common European diseases, resulting in rapid disease spread and high rates of mortality. Diseases spread more quickly on islands and densely populated regions such as parts of Mexico and Central American than in the United States and Canada, where diseases spread more slowly. It is estimated that 80 percent to 90 percent of the Native American population was killed by disease after European contact.

The Extremely Devastating "Disease Frontier"

The most catastrophic disease to affect Native Americans was smallpox, a highly contagious disease that swept through Europe several times during the Middle Ages. Other devastating diseases included measles, influenza, yellow fever, typhus, cholera and the bubonic plague. Whooping cough and mumps, considered commonplace childhood diseases in Europe, also killed adult Native Americans.