Hopping on the opposite side of the fence:
An illness (one of many) that reappears and spreads silently, then manifests symptoms later on (days or weeks later) might be interesting. In this case, neither the character nor the player would know that the character has been infected until later on - after they have made contact with and possibly spread it to countless others. Something like this would need to be fairly rare though.. perhaps yearly or bi-yearly ortherwise it would get old and played out fast.
As for cure, perhaps the method could still be left up to the player as the original post suggested. A group could role play their own cure and occly administer a potion to clear the character of the illness... or for those not interested in the RP route, they could still go with the potion, beg Sharven for a healing, or whatever...
