Why did native american have a high death rate from european disease

There were an estimated 18 million Native Americans living north of Mexico at the beginning of the European invasion. Prior to the arrival of the Europeans, American Indians were remarkably free of serious diseases. People did not often die from diseases. As the European explorers and colonists began to arrive, this changed and the consequences were disastrous for Native American people. The death tolls from the newly introduced European diseases often reached 80-90 percent. Entire groups of people vanished before the tidal wave of disease.  

Explanation:

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.

Overall, hundreds of thousands of Indians died of European diseases during the first two centuries following contact. In terms of death tolls, smallpox killed the greatest number of Indians, followed by measles, influenza, and bubonic plague.

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