'time bomb' illness

Huntington's Disease? That was the first thing that came into my mind. MS could be an option too, for a progressive illness.

It's rare, but ALS can occur in males in their 20s - 30s. That may be another option.

Lupus, that's always the answer for everything on House.

Lots of cancers can be stable for long periods of time and then get serious in a matter of days.

Malaria comes to mind. The symptoms can come and go, and with them whatever level of complication you deem useful. There's also the possibility of permanent neurological and organ damage.

If you're okay with disability rather than death, there's Retinitis Pigmentosa (RP). It can eventually cause complete blindness, but its progress isn't predictable or consistent. Night blindness and tunnel vision are sometimes the first noticeable symptoms.

It's also something that wouldn't be unusual for a guy in their 20s to just discover that they have it, or for it to have first been diagnosed when he was a kid.

If you still want ideas-- cystic fibrosis, a (primarily) lung disease. It's chronic so pretty much everyone is diagnosed at a young age and takes some meds and stuff, but especially these days a person with it can be very well for a long time, not disabled at all. But it can do downhill really, really quickly. A person with CF can go from totally fine to the ICU in a few weeks with a bad infection... and depending on the person and their ability to bounce back, to discussions of feeding tubes and oxygen and lung transplant within a couple months. As CF advances you can be quite disabled, breathless even with a few steps, constantly exhausted, malnourished, tons of coughing, frequent hospitalizations... until you essentially NEED a lung transplant or else. And as I said, this can progress even within a year. For people who've been quite healthy with it the 20s are often a time of serious decline, though it can happen any age. The median life expectancy in the US is about 37 right now.

If you are interested in exploring this route, do PM me-- as you may have guessed, I'm lucky enough to have CF. Best of luck with whatever you choose!

Heart defect. I lost a friend to this years ago. It was something he knew about all his life. One day he thought he had the flu, checked with his doc, who agreed it was the flu, then went home and died that night. Until that day, he was his normal self and feeling fine.

Leukemia (any variety) that is currently in remission... ominous and inevitable - a dark cloud that haunts your character.

Hemophilia - he'd have known about it, but any accident causing bleeding could be very dangerous.

All of life lies between the benign and the lethal. Any disease might work.

I carry a genetic marker that suggests I might have a greater risk for Parkinson's disease. My grandfather had Parkinson's and went through a very slow, gradual decline. I am very healthy, but I know that the symptoms for Parkinson's could begin to manifest at any point.