How infectious diseases helped the Old world conquer the new

Here are a few paragraphs from Cochran and Harpending in regard to the ease with which the Spanish conquered the Amerindians of America: 

“The Amerindians migrated from Northeast Asia some 15,000 years ago. They did not carry with them crowd diseases that arose after the birth of agriculture, nor did they carry the genetic defenses that later developed against those diseases. Since their path to the New World went through frigid landscapes like Siberia and Alaska, they left behind some of the ancient infectious diseases that were vectorborne or had complex life cycles—malaria and Guinea worm, for example. . .”

“Although Amerindians did develop agriculture independently—a very effective agriculture that included some of the world’s most important crops, such as maize and potatoes—they domesticated few animals, mostly because they had already wiped out most of the species suited to domestication. . .”

“. . . infectious disease was so unimportant among Amerindians, selection most likely favored weaker immune systems, because people with weaker immune systems would be better able to avoid autoimmune disorders, in which the immune system misfires and attacks some organ or tissue. Type 1 diabetes, in which the immune system attacks the pancreatic cells that make insulin, and multiple sclerosis, where it attacks the myelin sheaths of the central nervous system, are well-known examples—both are rare among Amerindians. A less vigorous immune system would have been an advantage under those conditions.

This Amerindian vulnerability was a primary reason for European success in the Americas. Epidemic disease, particularly smallpox, interfered with armed resistance by Amerindians and thus played an important part in the early Spanish conquests. In Mexico, where Hernán Cortés and his troops had made the Aztec emperor their puppet, the Aztecs rose against them, killing Moctezuma II and two-thirds of the Spanish force in the famous “Noche Triste.” The Aztecs probably would have utterly destroyed the invaders, were it not for the smallpox epidemic under way at the same time. The leader of the Aztec defense died in the epidemic, and Cortés and his men conquered the Aztec Empire. It is hard to see how Cortés could have won without those microscopic allies, since he was trying to conquer an empire of millions with a few hundred men.

Cochran, Gregory; Henry Harpending (2009-01-27). The 10,000 Year Explosion: How Civilization Accelerated Human Evolution . . .. Basic Books. Kindle Edition.

Comment:  We’ve known about the conquering of the Amerindians by the Conquistadors for a great many years, but what we didn’t know until scientists began working with the human genome (completed in 2003) was that the Amerindians didn’t have the diversified HLA systems.  In the Old World with all its years of agriculture since 8,000 BC, humans were subjected to a variety of diseases from animals, poor hygiene, and the close proximity they were to each other in cities.  Having different HLA alleles expands the range of pathogens that our immune systems can deal with.  Amerindians didn’t have that diversity.  Many tribes had only a single HLA allele.  

This also explains why the British had such an easy time colonizing North America.  The Amerindians had been decimated by disease.  The New World was largely empty.