Ehhh... while it might add to the 'realism', I know personally I'd be like ''WHAT THE %#@$'' if I go to sleep after fighting an enemy and it says ''You are now a vampire''. While it could be cool to not have a ''You have contracted [x]'' message, I say it should always appear in the effects menu, even if the 'symptoms' are slow to appear. The first time I ever died in Oblivion was because I was a vampire, and I was trying to cure myself and got stuck in the frickin' wilderness in full sunlight.
As for diseases having to have specific quests to cure them, or being permanent or any of that - I say no, just no. Vampirism can have a quest to cure it, since that's a ''major'' disease, but for anything else, it'd just ruin it.
I don't want to have to run to town or do some damn quest every 5 hours of gameplay because I have this disease now or that disease, and ''oh, you're now dying slowly, or can't do this that and the other thing because YOU HAD [x] DIEASE AND YOU DIDN'T KNOW! HA HA!''.
Just... no.
*shudder*
At first I have to say it happened to me on Oblivion. I was playing mainquest through with my first character and got azuras star for that. I didn't know I had a vampire disease before I wake up at the next morning as a vampire. Not nice surprise.
But what I mentioned about specific cure wasn't quest. I mean there should be different medicine for every disease. For rare diseases you have to make it by yourself from the special herbs or hire healer with high price to do it for you, but common medicines are easier and cheaper to find.
And this is not a part of the original game is just an idea for a mod and if you don't like it I think you shouldn't use it.