The main problem we were going to have is that they realized it was hopeless and got discouraged from completing their mission. If you have players that want to save the world (instead of just getting treasure and kill the monsters), I think you need to have an altruistic motivation.
The Death Curse essentially drops a character who has been raised's HP maximum by 20 immediately, then 1 hp per day until they're dead. The party's patron has 79 days remaining. Nearly half of those remaining days would be taken by walking through the jungle - assuming no encounters, every check is perfect, no dungeon exploration, and that they know exactly where to go from the beginning (which they don't). It's very improbable that they can save their patron (or anyone else struck by the Death Curse).
I'd either not do the 1 hp per day element, or give them a saving throw to reduce the chance. More than likely, what I'd actually do is to not have the Death Curse trigger until they are in the Yuan-Ti Temple or something like that. Have them know where they're going and have a reasonable chance to stop it.