how about a smoker? they look normal but are suffering a looong death... or how about syphillus (sp) ??takes along time, gets ya from the inside, dont notice anything until your eye sight goes...... the virus known to effet cleveland browns fans ( I am one so dont get all worked up ) you look normal, act like a fan, get your heart broken every year, there MIGHT be a cure but its a bit less than 50-50 call it the nfl eastern division I aint had a winner in years virus....

mutiple sclerosis

Hepatitis C transferred to him while inducted into the Army, the jet gun transmission of viral contaminants was in the range of 8-9 % from the sixties through the seventies. Conventional treatment is from 45-50% success rate, which leaves the question in grave doubt from the perspective of the one facing those odds. Cirrhosis develops and you waste away, a couch potato for years and then suddenly a bed ridden shadow of memories and unfulfilled dreams.

To clear up a little debate we're having on some comments, could you clarify exactly how slowly you want the person to die? Are we talking weeks? Months? Years? Decades? At the very end, when it's death or a possible miracle cure, can the person be showing symptoms, or still nothing that couldn't be explained away by stress, or a cold--minor weight loss, paleness, that stuff? That clarification would really help. Also, man or woman? How old? Also, does it have to be exotic or can it be mundane? And can the character have some small symptom that's easily not noticed or blamed on something else. Like the character is a little short of breath but he had childhood asthma and when to moved to ____, it flared up a bit. Or he isn't in great shape, keeps meaning to go the the gym, start a workout routine but somehow never does. Some sort of symptom like that that doesn't really concern people but they ask politely and they get some random piece of personal information (that the didn't necessarily want) about childhood, or information about someone's workout routine or lack thereof, that sort of thing and the moment's over, every continues on with their day, yada yada. And to everyone else it's a quirk that's now known about the character the way they know that so-and-so chews on pen caps every time he gets dumped. So is that ok?

a heart murmur can often be a missed condition that can kill someone during times of increased stress or strain on the heart IE:first time running a 5k. It can be treated if caught and a pacemaker is installed. The condition that is literally known as the "silent killer" in the middle community, (and I am in medicine) is high blood pressure. It can cause lots of serious heart damage over time that goes unnotices until the situation hits boiling point. It is one of those issues that will lead to many more fatal ones down the road. Let me know if you need more help

There is a rare inborn disease called Myasthenia Gravis that slowly paralyzes your body, makes you go insane and eventually goes to your respiratory cavity and kills you if untreated. However, the person looks normal on the outside. I think that would be a great illness for your character.