The setting is modern day Japan.
The character involved is a 24 year old ex-computer programmer/hacker turned into a delinquent school janitor. He's a smoker, a drinker, has had a past, brutal head injury involving a desk falling out a second story window, is generally under a wild amount of stress (keeping his identity hidden, constant guilt over the death of his friend, looking after his deceased friend's younger sister and keeping her out of danger) and experiences quite the amount of emotional turmoil during the course of the actual series.
I would like for him not to even know that he's sick until it happens - I want him to be going about his day before he starts to feel sick and eventually collapse. I would prefer for him to live, even if he's permanently damaged/needs surgery/needs a pacemaker/his lifespan is reduced.
I have googled "congenital heart defects", "heart conditions in young men", "heart conditions" and " heart diseases", and I've gone to Mayoclinic and webmd, but have come up short with ones that suddenly pop up in the mid twenties.
I appreciate your time! :)
ETA: <3 I've got some really awesome responses; thank you, everyone!!