In 1995, on a whim, I asked a friend: Which would worry you more, being attacked with a biological weapon or a chemical weapon? He looked quizzical. "Frankly, I'm afraid of Alzheimer's", he replied, and we shared a laugh. He had elegantly dismissed my question as an irrelevancy. In civilized society, people do not think about such things.
Leonard A. Cole in Scientific American
Unlike chemical weapons, biological weaponry has not been used extensively in human history. The first known incident occurred in the 14th century, when an army besieging Black Sea port Kaffa catapulted plague-infected cadavers over the city walls. The only confirmed instance in the 20th century was Japan using plague bacteria against China in the 1930s and '40s. Gruinard Island (off the coast of Scotland) remained infected with anthrax spores for 40 years after biological warfare tests were carried out there in the 1940s.
The infrequent use of biological weapons to date can be explained in multiple ways:
In 1972, the Biological Weapons Convention was signed by a large number of countries, thus prohibiting the development or possession of these weapons. Still in 1995, 17 countries were suspected by the CIA to develop biological weapons: Iran, Iraq, Libya, Syria, North-Korea, South-Korea, Taiwan, China, Israel, Egypt, Vietnam, Laos, Cuba, Bulgaria, India, South-Africa and Russia. Iraq has acknowledged the claims of United Nations inspectors that during the Gulf War it possessed Scud missiles tipped with biological warheads.