History teachers used to teach that North America had originally been occupied by one million Indians. That low number helped justify the white conquest of what could then be viewed as an almost empty continent. However archeological research excavations and descriptions left by the first European explorers suggest an initial number of around 20 million. In the century or two following Columbus's arrival in the New World, the Indian population is estimated to have declined by about 95% ! Part of the reason for this quick and overwhelming conquest goes back to the invaders' technological advantages. Europeans's guns and steel swords were more effective weapons than Native American stone axes and wooden clubs.

Only Europeans had ships capable of crossing the ocean and horses that could provide a decisive advantage in battle.
But that is not the whole answer. Far more Native Americans died in bed than on the battlefield -the victim of germs, not of guns and swords. Those germs undermined Indian resistance by killing most Indians and their leaders and by demoralizing the survivors.
The Indians had never been exposed to European germs and had neither immunologic nor genetic resistance to them. Smallpox ,measles , influenza , and typhus competed for top among killers. Pertussis, plague, tuberculosis, diphtheria, mumps, malaria, and yellow fever came close behind.
And while over a dozen major infectious diseases of Old World origin became established in the New World, strangely enough, not a single major killer reached Europe from the Americas !(The sole possible exception is syphilis, whose area origin still remains controversial.)
The reason behind this one sidedness appears if we know that crowd disease evolved from diseases of domesticated herd animals. There were many such animals in Eurasia. But there were only five animals that became domesticated in the Americas: the turkey , the guinea pig, the llama, the duck and the dog. And these few domesticated animals were unlikely sources of such diseases.
The importance of animal-derived diseases for human history extends far beyond the Americas. Eurasian germs played a key role in decimating native peoples in many other parts of the world as well, including the Pacific Islands, Australia and Southern Africa.