Biological weapons are sometimes called weapons of mass destruction (WMD) because they can kill huge numbers of people with a single use. For example, a 1993 study conducted by the U.S. Congressional Office of Technology Assessment found that spraying 100 kilograms (220 pounds) of dried anthrax spores over Washington, D.C., would cause between 1 million and 3 million deaths.

Biological warfare began centuries ago, when humans first realized that illnesses could be spread by diseased people. Some of the earliest biological weapons were diseased corpses, flung into an opponent's wells or camps to sicken enemy populations. In 1346, for example, Tatars attacked Caffa, an Italian trading post in the Crimea, on the Black Sea. During the siege the Tatar army was struck by bubonic plague, which killed thousands of soldiers. The Tatars turned their dead troops into weapons, catapulting disease-ridden corpses over the walls of Caffa...