Biological or germ warfare has a long history. For example, in the year 1343, Tatars (originally a nomadic tribe of east-central Asia) became sick with the bubonic plague. The disease, which is carried by fleas and rats, was called the Black Death because nearly all who became afflicted died. Invading the Crimea (in present-day Ukraine), the marauding Tatars encountered a group of Genoese (Italian) merchants at a trading post. Besieging them, the Tatars catapulted their dead at their enemy, many of whom became infected, carrying the plague to Constantinople (present-day Istanbul, Turkey) and to the western European ports where they traveled.

In the twentieth century, the use of microorganisms or toxins that produce sickness in people or in animals, or that cause destruction to crops, was outlawed by the Geneva Gas Protocol of 1925. In 1972 the Biological and Toxin Weapons Convention was simultaneously opened for signature in Moscow, Washington, and London, and the agreement entered into force on March 26, 1975. Signed by more than 162 nations, the convention bans “the development, production, stockpiling, acquisition, and retention of microbial or other biological agents or toxins, in types and in quantities that have no justification for prophylactic, protective or other peaceful purposes.” Nevertheless, several nations have conducted further research into defense against biological warfare, including developing microorganisms suitable for military retaliation. The existence of such biological weapons—including anthrax and smallpox—remains a concern today. The possibility that Iraq possessed biological and chemical weapons of mass destruction was the primary reason for the U.S.-led invasion of that country in 2003.