Pestilence is contagious disease that spreads out of control, killing many people. Here are examples of some of the worst epidemics around the world.

AIDS

• Where: Worldwide
• When: Late 1970s-present
• AIDS (Acquired Immune Deficiency Syndrome) is a disease that destroys the body's immune system, its ability to fight sickness. This virus may have developed as long as 50 to 150 years ago, but it was not identified until 1981. The virus may take years to produce symptoms in an infected human being. The United Nations and the World Health Organization reported that a total of 25 million people had died from AIDS as of Dec. 2006.

Black Death

• Where: Western Europe
• When: 1347-51
• This plague, thought to be the Bubonic plague, spread throughout Europe, killing about half the continent's population. It was called the Black Death because of the black blotches that appeared on the victims' bodies. This plague was carried by infected fleas on black rats.

Influenza

• Where: Worldwide
• When: 1918-19
• This flu was a highly contagious virus that killed 20 million people throughout the world. Without effective medication to treat the illness, most people died of complications from the disease, like pneumonia. This pestilence, along with the Black Death, resulted in the highest number of deaths worldwide in history.