Epidemic 10: Smallpox

Before European explorers, conquerors and colonists began to flood into the New World in the early 1500s, the Americas were home to an estimated 100 million native people. During the centuries that followed, epidemic diseases decreased that number to somewhere between 5 and 10 million [source: Yount]. While these people, such as the Incas and the Aztecs, had built cities, they hadn't resided in them long enough to breed the kind of diseases Europeans had, nor had they domesticated as many animals. When the Europeans arrived to the Americas, they brought with them a host of diseases for which the native peoples had no defense or immunity.

Chief among these diseases was smallpox, caused by the variola virus. These microbes began affecting humans thousands of years ago, with the most common form of the disease boasting a 30 percent mortality rate [source: CDC]. Smallpox causes high fevers, body aches, and a rash that develops from fluid-filled bumps and scabs to permanent, pitted scars. The disease predominantly spreads through direct contact with an infected person's skin or bodily fluids, but can also be spread though the air in close, confined environments.

Despite the creation of a vaccine in 1796, smallpox epidemics continued to spread. Even as recently as 1967, the virus killed two million people and scared millions more across the world [source: Choo]. That same year, the World Health Organization spearheaded an effort to eradicate the virus

through mass vaccinations. As a result, 1977 marked the last naturally occurring case of smallpox. Effectively eliminated from the natural world, the disease exists only in laboratories.

Learn how and why 1918's influenza strain killed millions of people on the next page.