Contact between Europeans and Native Americans led to a demographic disaster of unprecedented proportions. Many of the epidemic diseases that were well established in the Old World were absent from the Americas before the arrival of Christopher Columbus in 1492. The catastrophic epidemics that accompanied the European conquest of the New World decimated the indigenous population of the Americas. Influenza, smallpox, measles, and typhus fever were among the first European diseases imported to the Americas. During the first hundred years of contact with Europeans, Native Americans were trapped in a virtual web of new diseases. European diseases, seeds, weeds, and animals irreversibly transformed the original biological and social landscape of the Americas. By 1518, the Native American demographic catastrophe and the demands of Spanish settlers for labor led to the importation of slaves from Africa. Thus, the Americas quickly became the site of the mixing of the peoples and infectious agents of previously separate continents.
Despite considerable progress in analyzing traces of the early migrations to the Americas, there is still doubt about the time of the arrival of the first humans. Some scholars believe that wandering bands of hunter-gatherers first crossed a land bridge from Asia to the New World about 10,000 years ago. Other evidence suggests that human beings might have arrived much earlier, but the earliest sites are very poorly preserved. In any case, migration from Siberia to Alaska might have served as a “cold filter” that screened out many Old World pathogens and insects. In addition, except for the late development of a few urban centers, primarily in Mesoamerica, population density in the New World rarely reached the levels needed to sustain epidemic diseases.
Centuries before Europeans arrived in the Western Hemisphere, advanced cultures and great cities had developed in Guatemala, Mexico, and the Andean Highlands. These areas were not free from disease, but accounts of pre-Conquest epidemics were generally associated with famines. Archeological evidence suggests that there were several periods of significant spurts of population growth and sudden declines in the Americas long before European contact. However, the impact of European diseases and military conquest was so profound and sudden that other patterns of possible development were abruptly transformed. Contact events involving the Aztecs, Mayans, and Inca civilizations were especially dramatic, primarily because Mexico and Peru had the highest population densities and the most extensive trade and transport networks in the Americas. Such factors provide ideal conditions for the spread of epidemic diseases.