Simply put, they all have been used in dispersing biological weapons.
At the “Battle of Tortona” in Italy in 1155, Barbarossa had human body parts dropped into the wells of his enemy to contaminate their water supplies.
Catapults were used to send dead and infected bodies of plaque victims over castle walls to infect the inhabitants under siege. In 1346-47 the Mongols used this tactic successfully at the city of Kaffa (in Crimea), forcing the Germans to flee.
During the French & Indian Wars in the USA 1763, blankets from patients that carried the smallpox virus were given to American Indians by British troops to infect and kill them.
In 1940 the Japanese took rats that were infested with plaque carrying fleas and set them loose in China causing epidemics.
In 1978 while living in London, a needle on the tip of an umbrella was used as a weapon to assassinate Georgi Markov. The neelde injected a pellet laced with lethal ricin toxin in to the diplomat.
And in 1984 a radical group in Oregon state laced the salad bar of a local restaurant with salmonella in order to make the population sick so they couldn’t vote against their candidates in a local election.
And then there are the anthrax letters of September 2001…..