"Not dying" isn't a reward, it's just avoiding a failure state.
Now, I don't have a problem with high-level diseases that require an epic quest to cure. The character should always have something that threatens them, and there should never be any guarantees. But the more it's based on pure luck, the more upset the player will be if they fail and the less accomplished they will feel if they succeed (If it's down to a coin toss, you aren't good or smart, you just got lucky). There should definitely be some leeway - Plenty of warning that something's wrong, enough hints to help you figure out what you need to do (Even if they're vague or intuitive - You're sick so you decide go to hospital, get tested with a machine there, which references a military base, where you find a message that gives the pass code to a previously impenetrable/hidden super laboratory, where the player finds the cure or the ingredients/gear to craft the cure), and enough time and then some to accomplish the task.
Remember, even while the player's trying to cure this disease they have a number of other things that can kill them, particularly if the disease lowers the character's capabilities. Time should only be a factor if the player ignores the problem entirely or doesn't act wisely during the course of the disease (Eating jellies and antibiotics to keep it at bay and mitigate symptoms, for example).
Once the player gets to wherever the cure is, there should be other things to keep them occupied. There could be some nifty CBMs, a stash of rare crafting materials, a robot companion NPC, a quest-line they can follow, and/or even a way to turn on "Hard Mode", like defeating the Wall of Flesh in Terraria. This gives players incentive to progress with the game by catching the disease rather than avoiding it completely, and rewards them for actually going through with it.
And the player should be immune to that particular super disease after that. It'd be silly to go through all that just to get it again.