Historians estimate that the Native American population at the time of Columbus' first landing was approximately 50 million, and this population decreased by as much as 90 percent by 1700. Historians consider the spread of disease, due to a lack of genetic diversity and contact with Europeans and Africans, to be responsible for killing between 50 and 90 percent of Native American populations during this time.
By 1600, disease had already decimated the Native American populations of Latin and South America. For example, two separate epidemics of viral hemorrhagic fever in 1545 and 1576 claimed the lives of between 7 and 18 million or up to 85 percent of the Mexican population, and syphilis, malaria and smallpox had eradicated many of the tribes of South America. Successive waves of smallpox, bubonic plague, typhus and yellow fever swept through different regions at various points in the 17th century. Other epidemics originated through contact with domesticated animals, such as cholera outbreaks associated with the contamination of drinking water by cattle. These epidemics sometimes wiped out entire nations, such as the Carib and Arawak.
In some instances, the spread of disease preceded the arrival of European settlers. For example, the tribes of Massachusetts and other parts of New England experienced epidemics that killed up to 90 percent of the indigenous population between 1600 and 1620, before the Puritans and other groups colonized the region. Scholars attribute these epidemics to direct or indirect contact with the French and Dutch traders.
Other causes for the decline in Native American populations include warfare with Europeans, and among themselves, and the intermingling of European and Native American races. For example, Spanish settlers, who were exclusively male, tended to marry the women of local tribes whose male populations had been killed in battle or by disease.