So, one day my party and I were walking through the woods, just having a grand ol' time. I was a Wilden fighter, spec'd for sword-and-board. Suddenly, an owlbear leaped out of the bushes, and after technically putting my flat-footed fighter down in a single hit, the DM intervened and I instead survived the encounter.
Well, most of me did. We then spent the next few sessions seeking out a way to regenerate my severed shield arm, and in the meantime half of my build's powers and abilities were either useless or significantly less effective.
The point being that if some terrible ill like this has befallen the party, turn it into an adventure/plot hook all on its own. Make a big deal out of what really is a big deal. Drop some hints about where the characters can go, what they can do, to find/make a cure. Send them on a quest to solve this problem.
And in the meantime, be a good GM and take into account your party's makeup when planning future encounters, but don't make it obvious that you are. Does this fight call for a dozen minions? Make it 8 instead. Is this supposed to be a L12 Elite? Make it L11.
The entire point of the game is to overcome challenges. Usually these are fights or skill challenges, but diseases and curses are challenges to be defeated, too! If it's a big enough deal, though, remember to hand out XP for overcoming it! ;-)