Biological warfare is defined as any living organism (such as contagious disease bacteria) used to harm an enemy's troops, livestock, crops or civilians. From bubonic plague to anthrax, many diseases have been used in this manner and many means of dispensing these diseases have been developed, from airborne aerosol to catapults flinging corpses, or even just spreading the disease yourself. This tactic of germ warfare has been used for centuries, and will probably be used for centuries to come.
In the medieval era, warlords built huge castles and forts with walls many feet thick to protect their interests from rivals. Often, because the wall were so impenetrable, a king or other ruler could hoard supplies and stay inside the walls for months or even years during a siege. Many times the warlord on both sides would be drained of resources after the siege and would be vulnerable to attack by other military leaders.
To try to stop these long ordeals, the attacker would fling plague-ridden corpses over the castle walls to infect the people inside. Huge catapults and trebuchets were used to do this. Sometimes, entire cities were wiped out because of this, and the surrounding country infested with plague.
Later, during the early American colonial times, thousands of European settlers swarmed over the Atlantic Ocean searching for a new life. They brought livestock crops and other essentials with them; they also brought cats, rats and diseases. The Europeans, many times had a built up immunities to these diseases, and so were relatively unaffected, since they already had these diseases in their old countries. The Native