sry draco, but i don´t think its ok...
while not worsen on less than a 2 for a very high constitution guy with endurance trained or a 5 for either trained in endurance or having 18 constituton is reasonable, or never worsen when you are treated by a wise healer, improving on your own is too easy.
see my above post for what seems to be correct ---> basic model for success/fail/critical failure is the death saving throw:
20/19-10/9-1 -> 3 failures result in death when not treated by a healer or luck!
This model would make a diseases not too deadly, also not trivial and make cure disease ritual worthwile and not more deadly than just using a heal check...
The difference is that, unlike with death saves, you start with a bad effect from the disease, so maintaining it isn't really 'good' in the first place, and the first failed "save" makes it worse. With death saves, you can have 2 failed saves without it being a big deal ... it's only the 3rd fail that kills you, similar all the damage in the world doesn't really hurt you except for the one that knocks you below 0HP.
The initial disease condition isn't
too nasty, but is still something you'd rather get rid of than just maintain. If it gets worse though, it can be really bad.
Some examples out of the DMG: at level 12 there is cackle fever which causes you to start the day without an action point and you are dazed (save ends) whenever you become bloodied. Maintaining that effect isn't a good outcome. If it gets worse you can't gain or use action points.
With diseases it's go back to normal (or closer to normal, some diseases have steps between cured and their initial effect), stay with a bad effect, or get a worse once. It's good, bad and ugly. Maintaining is still a failure, just not as spectacular a failure as worsening is. It might make removing disease a less commonly used ritual, but with some of the effects the diseases have, the option of eliminating a disease in 1 hour is a bit better than having to continue with the penalty for the rest of the day.
Also, consider that, it's quite possible for most of the party to end up with the disease because of the source being a monster they were fighting.
(a) to use a heal check in place of an endurance check you have to be attending to the infected. So, that means the person with the great heal check won't be able to help
himself against the disease. He'll be relying on his own endurance and/or someone elses heal check.
(b) If you are making rolls for 5 characters, there is a chance that once of them may still end up being low enough to worsen, and at the very least, it will be unlikely that everyone will succeed on being cured/improving.