This reminds me of the campaign section I just finished for my 4th ed group.
They were all level 12-13 and I still managed to threaten them with the horribleness of disease! At one point the barbarian had over 70 bites. They then wasted all their cure disease scrolls on fixing him. They didn't wonder ontot the streets again after that! It made it very tense as they tried to find resources and save the city folk!
Keep up the good work though, It sounds like your having fun, which is the important bit!