People at war have been making use of germs and animals for millennia. Central Americans would coat darts with poisons, Assyrians created epidemics in enemy lands by forcing emigration of ill people, Hannibal of Carthage was known to wedge venomous snakes in clay pots and have his men hurl the fragile bombs onto enemy ships during naval combat. If there is anything that humanity has gotten really good at over our stay on the surface, it’s our ability to weaponize things.
There are accounts of Marcus Aurelius poisoning the wells of his enemies, slowly removing the bare necessities of life one piece at a time. The phrase “salt the earth tactics” is generally considered to be a relatively common idea. It hails back to the original style of biological warfare (back before people knew what germs were) with the idea of plowing the earth and spilling salt into the soil to prevent any plant life from growing there. It turns out that there are no actual accounts of “salting the earth” in history, most of them come from folk lore or from spun myths of victorious generals. In a way, it makes sense that nobody ever salted the fields to prevent their foes from resurging because salt was exceptionally difficult to make, incredibly useful and opulently expensive. Emptying out a hundred thousand drachma just to prove how much you won a battle was never really a thing. The origin of the salting likely came from Hittite rituals where salt would be sprinkled over conquered lands to purify the area.
As for biological warfare, humanity has a long history of using bioweapons and bioterrorism. Mongolian sieges on Russian (Tartar) lands were punctuated by stories of the Golden Horde catapulting corpses of those who died from the bubonic Plague into enemy castles, creating outbreaks inside the besieged zones. Well before people began to understand truly how illnesses worked, Medieval doctors believed that the plague was caught by smelling or inhaling the miasma of the ill and dying. Doctors during the 1300s would wear strange masks with a bird-like beak that they would fill with perfumes and flower petals to prevent them from smelling the Black Death. Europeans later took similar tactics to the Mongolians, as they would trebuchet dead and rotting cattle into enemy castles, spreading disease and nastiness wherever the bovine missile connected. Some accounts even exist of European generals catapulting cartloads of dung over the walls of their quarries.
English longbowmen were known to jab their arrows into the ground in front of them rather than carry them in a quiver; although unintentional, the arrows would carry moist dirt in their serrations and have a substantially higher infection rate than arrows that were sanitary. American military leaders infamously gave American Indians blankets that had been used by people who were infected with smallpox. As germ theory became more solid, biological warfare began to take a more dramatic turn towards the deadly.
Perhaps one of the more interesting stories that history provides about biological warfare comes from France, when Louis XIV reportedly turned away an Italian chemist who had created the ultimate weapon. The stories are that Louis refused to use the tool, instead paying the chemist a healthy salary on the condition that he never divulged the secret of how to use or create the weapon.