There were a number of factors that reduced the Native American population in the centuries after the arrival of Europeans in 1492. Whites killed Indians during wars of conquest or shortened their life-spans by enslaving them or radically altering their environments. But the chief factor that decimated the Indian population in North America was the diseases brought by the Europeans.

A Large Pre-Contact Population

In determining how many Native Americans died from European diseases, scientists and historians have had to approximate how many Indians there actually were in North America pre-European contact. Before the middle of the 20th century, estimates tended to be on the low side, with the U.S. Census Bureau in 1884 believing that there were, at most, 500,000 Indians. Today, there is a rough consensus that the populations were much larger, perhaps 12 million Indians living north of the Rio Grande, with more millions in Central America, South America and the Caribbean.

No Immunity to Disease

This makes the slaughter wreaked by the Europeans’ prime killer, smallpox, all the more appalling. Smallpox, to which the native population of the Americas initially had no resistance, came to the island of Hispaniola (today’s Haiti and Dominican Republic) between 1507 and 1517 and probably killed off a third of the Indian population. Within a generation after Columbus’ first visit to the island in 1492, only 11,000 Indians remained from an estimated pre-contact three million. Smallpox came to Mexico and Central America with the Spanish in 1520, so decimating the Mexica that they were unable to fight off conquest. By 1527, it sped south ahead of the arrival of Francisco Pizarro and his men to the great Inca Empire in Peru, killing 200,000 of an estimated six million inhabitants, and making their conquest by Pizarro in 1533 much easier.

A Terrible Morality Rate

Smallpox arrived in New England a year or two ahead of the Pilgrims in 1620, seriously weakening them, with the disease often destroying entire villages. As survivors fled west, they brought smallpox with them to other tribes, killing Indians who had not yet had contact with whites. Tribes throughout North America were repeatedly decimated by waves of smallpox during the course of the 17th century, with nearly half of the Cherokee population dying from the disease. By the end of the 18th century, Indians in the northeastern part of the United States had suffered mortality rates of as high as 84 to 95 percent of certain populations, with a general mortality rate of about 55 percent.

New Villages Built Over Old Ones

Smallpox was not the only European disease the Indians fell prey to -- they also lacked an immunity to such illnesses as typhus, diphtheria, measles, mumps and whooping cough. To what extent disease decimated the Native American population will never be known with exactitude, although anthropologist Henry Dobyns has controversially claimed that it was as high as 95 percent. His figures are disputed, but certainly millions of Indians lost their lives to European illnesses, paving the way for the arrival of Europeans to the New World. In New England alone, more than 50 of the first colonial villages were erected on the site of Indian villages deserted because of smallpox.