European explorers to the Americas between the 15th and 19th centuries brought several diseases with them that proved deadly to the native population. Diseases such as smallpox, influenza and measles killed approximately 90 percent of the Native American population. The indigenous people did not have any previous exposure to these deadly diseases, and had no natural immunity. Sometimes the illnesses spread after direct contact with European settlers, often resulting in deadly outbreaks that decimated entire villages. In the eighteenth century, smallpox outbreaks during the French and Indian and American Revolutionary Wars killed more than 100,000 Native Americans.

Early European Contact

Several European diseases, including smallpox, were introduced to the indigenous peoples of North America by the Spanish and Portuguese conquistadors of the late 15th and early 16th centuries. In fact, historians report smallpox was instrumental in the fall of the ancient Aztec and Inca empires. Smallpox, which was also introduced by French and British settlers, is similarly said to be responsible for the severe reductions of the Wampanoag and Abenaki peoples of modern New England, as well as the tribes of the Iroquois Confederacy. The death toll among just one of those tribes, the Mohawks, during a 1633 outbreak, reduced the tribe’s population from 8,500 to fewer than 2,000.

French and Indian War

Smallpox was the weapon behind one of the first known cases of biological warfare. Sir Jeffrey Amherst, the commander of the British Army in North America, reportedly suggested contaminating the native tribes who were hostile to British forces with the virus during the French and Indian War (1754-1767). The British Army reportedly used smallpox-contaminated blankets to spread the virus among natives, resulting in an epidemic with an estimated 50 percent mortality rate.

Revolutionary War

Smallpox made its way to Boston by the early conflicts of the American Revolutionary War in 1775-1776. The infection spread to Canadian indigenous tribes during the Continental Army’s invasion of Quebec. In a letter to his wife Abigail, founding father John Adams wrote of the horrors of smallpox, but added that it was a “small consolation” to know that the “scoundrel savages” fighting against American forces had been infected with the deadly disease. The ailment spread West to the Iroquois, before eventually making its way South and contaminating the native tribes surrounding Mexico City, Texas and New Mexico by 1781.

Drop in Native Populations

The disease (in addition to warfare and enslavement) that Native American tribes met as a result of contact with European settlers resulted in a 50 percent population drop shortly after 1500 -- almost a decade after Christopher Columbus arrived in the New World. The sudden, extreme decline indicates the population reduction was almost exclusively the result of infectious diseases sweeping through native communities, according to a 2011 study published in the Proceedings of the National Academy of Science.