Aztec Smallpox was the terror of the European world. And it was even worse in unknown territory. The Aztec people who had never known smallpox were decimated when the illness was introduced by the Spanish Conquistadors under Hernando Cortes. With only about 500 men, Cortes quickly and thoroughly destroyed an Aztec civilization of millions based on a large part to smallpox. One of the lessons of the unfortunate Aztec experience was that microbes, and the diseases they cause, can travel across the world. And when a disease hits a population that has never encountered it, the results can be devastating.