There were an estimated 18 million Native Americans living north of Mexico at the beginning of the European arrival. The death tolls from the newly introduced European diseases often reached 80-90 percent. Entire groups of people vanished.

The diseases brought to this continent by the Europeans included bubonic plague, chicken pox, pneumonic plague, cholera, diphtheria, influenza, measles, scarlet fever, smallpox, typhus, tuberculosis, and whooping cough. The diseases introduced in the Americas by the Europeans were crowd diseases: that is, individuals who have once contracted the disease and survived become immune to the disease. In a small population, the disease will become extinct. Measles, for instance, requires a population of about 300,000 to survive. If the population size drops below this threshold, the virus can cause illness and death, but after one epidemic, the virus itself dies out.

Another important factor in the European diseases was the presence of domesticated animals. The source of many of the infections was the domesticated animals which lived in close proximity with the humans.

Why were the Native Americans free of these diseases? The two largest reasons may be the following ...

First, Indians had better diets and they were less likely to face starvation and hunger. The first Europeans to reach North America often commented on the large size of the Indians. American Indians were larger than the Europeans simply due to better diets and less starvation. Unlike the Europeans, Indian political leaders did not store their wealth but accumulated prestige by giving food to those in need. No one in an Indian village or an Indian band starved unless all did so.

Secondly, American Indian populations did not have many of the infectious diseases that were endemic in Europe. A number of reasons have been suggested for this lack of disease. Some scientists have suggested that Indian people came to this continent through the cold, harsh climate of the north and that this acted as a germ filter which screened out infectious diseases. Others have suggested that Indians were disease-free because of the lack of domesticated animals. Measles, smallpox, and influenza are among the diseases which are closely associated with domesticated animals. Lacking the large domesticated animals, there were comparatively few opportunities in this hemisphere for the transfer of infections from animal reservoirs of disease to human beings.