The greatest adversary to the natives in the Americas was not the swords or guns of the invaders. It was the devastation brought by deadly diseases infecting an unsuspecting population that had no immunity to such diseases.
The Europeans were said to be thoroughly diseased by the time Columbus set sail on his first voyage (Cowley, 1991). Through the domestication of such animals as pigs, horses, sheep, and cattle, the Europeans exposed themselves to a vast array of pathogens which continued to be spread through wars, explorations, and city-building. Thus any European who crossed the Atlantic was immune to such diseases as measles and smallpox because of battling them as a child.
The original inhabitants traveled to the New World in groups of a couple hundred each. Because microbes such as the ones that cause measles and smallpox need populations of several million to survive, the original populations were unaffected by the deadly diseases. However, by the time Columbus arrived, the major Indian groups of Aztecs, Incas, and Mayas had built their populations up enough to sustain mass epidemics. Evidence shows that these populations suffered from such diseases as syphilis, tuberculosis, a few intestinal parasites, and some types of flu, but not the diseases that had been infecting the Old World for centuries. Thus when the Europeans arrived bringing diseases such as smallpox, measles, whooping cough, etc. the natives were immunologically defenseless (Cowley, 1991).
It is believed that 40 million to 50 million people inhabited the New World before the arrival of Columbus and the Europeans, and that most of them died within a few decades. For example, Mexico's population fell from about 30 million in 1519 to 3 million in 1588. The other South and Central American countries as well as the Caribbean islands suffered the same devastation (Cowley, 1991). Mass epidemics were virtually unknown in the New World prior to the invasion of the Europeans. Aside from their lack of immunity to the pathogens, another factor in the rapid spread of the diseases could have been the weariness of the Indian populations. Due to their recent conquest and oppression by the Spaniards, the Indians were probably too tired to fight the infections.
One thing that must be noted is that contrary to popular belief, infectious agents such as viruses, bacteria, parasites, etc. are not designed to cause harm. Rather they survive better when they don't destroy their hosts. The greatest harm occurs when a germ infects a previously unexposed population. It often causes massive epidemics of which only the most resilient individuals survive. As natural selection weeds out the most susceptible hosts and the survivors repopulate, what was once a deadly disease becomes a routine childhood illness.
The first major disease to find the New World was probably smallpox which broke out on Hispanola in 1518. As the Spaniards moved toward the mainland from the islands their diseases often...