Picture adapted from: http://www.rr.gmcs.k12.nm.us/domagala.abomb.htm
The history of Biological Warfare (BW), can be divided into 3 parts: Early History, Modern History, and what we call "The Now." The early history starts as far back as you can think and continues up to the start of the 20th century.
The first recorded use of biological agents is the Romans using dead animals to foul the enemies water supply. This had the dual effects of decreasing enemy numbers and lowering morale. The idea behind this kind of attack is that a weakened enemy is an easily defeated enemy.
The Tartars had the idea of infecting the enemy by catapulting bodies infected with bubonic plague over the walls of the city of Kaffa. Some historians believe that this event was the cause of the epidemic of plague that swept across medieval Europe killing 25 million.
A more 'recent' use of BW involves the British during the French-Indian War. The Native Americans greatly outnumbered the British and were suspected of being on the side of the French. As an "act of good will" the British gave blankets to the Indians, but the blankets came from a hospital that was treating smallpox victims and consequently smallpox raged through the Native American community and devastated their numbers.
(SOURCE: CALOPOLY WEBSITE)
Link to Biological Warfare Legacy