Let's make one thing clear:
Disease warfare in the medival timeperiod were the result of actual diseases. Many times, the besiegers were so devestated by the sickness that they had to call off the siege. As a last act, they would do things to spread the disease to the besieged - if you can't rule them, you might as well kill them - attitude.
An example of this is the Turkish siege that started the spread of the Bubonic Plague in Europe.