The second reason Native Americans were historically 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. The only ones who survived were those with a greater genetic ability to withstand those particular diseases. Throughout their history, Native Americans had very few plagues. And since plagues hadn't killed those who could not withstand the diseases, they had few, if any immunities to dangerous diseases.
But why didn't Native Americans have plagues? The main reason is that historically, most of the European plagues originally came from domesticated animals, and Europeans far more domesticated animals than Native Americans had. 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. Throughout European history, plagues came from close contact with an animal that 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 descendants from a very small group of people who came across the Bering Strait to populate the Americas only around 13,000 years ago. The gene pool was limited because it came from such a small group of people. Immune systems thrive on variety. The greater the variety in an 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.
And the fourth way they were more vulnerable is that their culture had so little experience with plagues and contagions, they had not historically developed good plague-preventing customs. 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 and, without knowing it, be infected with the microbes. After he died, his friends and family fanned out and infected others. This practice made contagions spread quickly.
Most people know something about Native American history. They know that lots of Native Americans died from disease. But new findings are showing the loss was much worse than we have historically been taught. The reason it hasn't been discovered until recently is that Europeans' first contacts with Native Americans were brief. Europeans, for the most part, touched down at coastal areas at first, and then went away, or only stayed near the coast. Of course, they unwittingly transmitted diseases that spread out from wherever they touched down, and those spread inland on their own.
Because of the four vulnerabilities listed above, the European 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 culture. And most of these Native American epidemics 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 had recently ruled the land, not with the splendor and wonder of intelligent cultures in full bloom, but with the last remnants of a catastrophe 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.
Adam Khan is the author of Self-Reliance, Translated, Slotralogy, and Principles For Personal Growth. Follow his podcast, The Adam Bomb.