Tatars in the fourteenth century hurled dead bodies foul with plague over the walls of enemy cities. British soldiers during the French and Indian War gave unfriendly tribes blankets sown with smallpox. I spread glanders, a disease of horses, among the mounts of rival cavalries. The Japanese in World War II dropped fleas infected with plague on Chinese cities, killing hundreds and perhaps thousands of people. Despite occasional grim successes, germ weapons have never played decisive roles in warfare or terrorism.
Unintended infection is another matter.
European conquests around the globe were often made possible because the indigenous peoples lacked immunity to the invaders' endemic diseases, including smallpox, measles, influenza, typhus, and plague. But intentional warfare with germ weapons has been relatively rare, especially in modern times, and has been widely condemned as unethical and inhumane. Even so, in the early twentieth century, Canada, France, Germany, Japan, the Soviet Union, and the United Kingdom were among the many countries that investigated how to wage biological war. All understood that the weapons they were developing were fundamentally different from bombs and bullets, grenades and missiles. These munitions were alive.