FOUR REASONS STAND OUT. Most of us know about the first two, but few of us know about the last two. First off, when a European child got the measles, he would usually live. And for the rest of his life he would be immune. Adults who get measles are more likely to die than children, so when adult Native Americans got the measles, it was more deadly. The European adults had an acquired immunity, but the adult Native Americans did not.

The second reason Native Americans were so vulnerable to European diseases is that the Europeans had gone through plague after plague, which wiped out large portions of the European population again and again. This was natural selection at work, and the only ones who survived were those with a greater genetic ability to withstand those particular diseases. The Native Americans had very few plagues in their history, so plagues had not killed off those who could not withstand the diseases.

Why didn't Native Americans have plagues? The main reason is that most of the European plagues originally came from domesticated animals, and Europeans had far more domesticated animals than Native Americans did. Thus more plagues. Thus more genetic resistance to those diseases. (Read more about that here.)

Native Americans had no cows, horses, chickens, pigs, goats, sheep, geese, lamb, oxen, or donkeys. European plagues came from close contact with an animal who had a disease. The animal microbe crossed over to humans and mutated into a human disease. Smallpox, for example, originally came from a cow. Cows get cowpox, which doesn't invade humans very successfully, but with continued contact, a strain developed that did invade humans: Smallpox.

The third way Native Americans were vulnerable is they are all descendents from a very small group. So the gene pool was small. The immune system thrives on variety. The greater the variety in the immune system, the greater number of microbes it can recognize and therefore destroy. The purpose of sex, from the gene's point of view, is to mix immune-system capabilities — to gain a greater variety in the offsprings' immunity responses.

Because the gene pool was so small, Native Americans had less variety in their immune responses, so were more vulnerable to disease.

The fourth way they were more vulnerable is that their culture had so little experience with plagues and contagions. Since plagues had plagued Europe for so long, Europeans learned to isolate sick people. They learned to try to contain the spread of the disease.

Native Americans didn't have this accumulated cultural knowledge. So when someone came down with smallpox, for example, all his family and friends would gather to his bedside. After he died, his friends and family fanned out and infected others. This practice practically guaranteed that the contagion would spread like crazy.

The tremendous loss of the cultures of Native Americans is much greater than people once believed. The reason it hasn't been discovered until recently is that Europeans' first contacts with Native Americans were brief. Europeans touched down at coastal areas at first, and then went away. Of course, they unwittingly left diseases that spread out from whereever they touched down, and spread inland.

Because of the four vulnerabilities, these diseases spread plague after deadly plague across the land. In a period of 130 years, something like 95 percent of all Native Americans died of disease. That number is far greater than experts (until recently) had ever suspected. The Native Americans who survived the plagues were, of course, completely demoralized and depressed by this tremendous loss of their loved ones, of their lifestyle, and of their ancient culture. And for most of these people, all of this happened before a European ever encountered them. So even first-hand accounts of "first contact" with inland Native Americans were not with the impressive cultures that recently ruled the land, not with the splendor and wonder of intelligent cultures in full bloom, but with the last remnants of a disaster on a scale we can hardly imagine.

If you'd like to read more about this, I recommend the book, 1491: New Revelations of the Americas Before Columbus, by Charles Mann.