Quote Originally Posted by Perp
Not exactly correct. First of all it was smallpox. Secondly, biological warfare was used during the middle ages (if not earlier) when attackers would lob diseased carcasses over castle walls with their catapults. Sickness would spread inside the castle forcing surrender.

The most infamous of these sorts of seigebreaking techniques was used in what was formerly Constantinople and resulted in a plague we now call the Black Death.