The Origins of Biological and Chemical Warfare

Chemical and biological warfare are not an invention of the 20th century.
Solon (638-559 BC) used a strong purgative, the herb hellebore, in the siege of Krissa. During the 6th century BC, the Assyrians poisoned enemy wells with rye ergot. In the Peloponnesian War (431-404 BC), the Spartans flung sulfur and pitch at the Athenians and their allies. In the Middle Ages, besiegers used the bloated and dripping bodies of plague victims as readymade "dirty bombs".
In 1346, during its siege of Kaffa (present day Feodosia in Crimea), the Tartar army suffered an outbreak of the Plague. They hurled the corpses of their infected dead over the city walls and into the city's water wells. The resulting epidemic led to the city's surrender. It is widely believed that people afflicted with the horrendous disease fled the place and started the Black Death pandemic which consumed at least one third of Europe's population within a few years. Russian troops adopted the same tactic against Sweden in 1710.
Smallpox was another favorite. Francisco Pizarro (1476-1541) gave South American natives clothing items deliberately contaminated with the variola virus. During the French and Indian wars in North America (1689-1763), blankets used by smallpox victims were given to American Indians. General Jeffery Amherst (1717-1797) gifted Indians loyal to the French with smallpox-contaminated bedspreads during the French and Indian War of 1754 to 1767. An epidemic broke among the Native American defenders of Fort Carillon and they lost it to the English.