Smallpox was the terror of the European world. And it was even worse for people who had never been exposed to it. New World peoples who had never known smallpox were devastated when the illness was introduced by European invaders. Smallpox killed around four million Aztecs and allowed Hernando Cortés to conquer their mighty empire with only about 500 men.

The Aztecs misfortune taught us that microbes and the diseases they cause, can travel across the world and devastate populations who have never been exposed to them.