Actually, that makes sense. Have the disease affect the besieged as well as the siegers. That way there is a definite trade off. The besieged will die faster [ or since I do not like that idea, loose more moral or a percentage of their units] than the attackers. A good idea - but it needs to be developed.
If it does not affect the sieger, than all strategy will go down the tubes and sieges will become a waiting game where the defender always loses.