Some of the diseases and poisons have long term effects. Like for example Blightburn. Look into those. Make it so they go to a cleric or have one of their own, but the cure fails. The party has to quest for the actual cure. You may have to take some GM Fiat into account and make your own up, but drained is a side effect of some of these diseases. They get weakened, pick a fight and get KO’d, maybe captured, or something.