It would be nice if you don't know if you're diseased - you just feel the affects of it. To see if you are diseased and which disease it is you need to talk with a healer. It would be also great if there's no standard medicine (cure disease potion) which cures all diseases. Praying on altar to cure all diseases was also too easy way to get rid of them. I'd like to see all diseases to be more like the corpus disease in Morrowind: it's affects continued growing and there was only a specific way to cure it.
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*