What I need, thus:
An ultimately fatal condition, which does not have terribly drastic symptoms until late stages, and which could plausibly kill a male around age 18. If an earlier symptom is moderate fatigue or similar, that's fine, as long as the guy can still at least occassionally sleep with his girlfriend. She's been burned before so if he needs to take it slow/easy because of his condition, she'd likely only find that romantic, so it can be fudged a little bit. This condition cannot be infectious, as, well, he'd be a jerk not to tell his girlfriend about it in that case and the idea is for her to have had at least one non-jerky boyfriend during her school years. I can work with him having good or bad periods, or with him taking medication to manage his condition, as long as it's reasonable for him to have been able to hide it from his girlfriend.
Ideally, this condition will typically be slow-acting enough that the guy can reasonably expect enough advance warning to put his affairs in order, but has the rare potential of very rapid decline leading to hospitalization-death. If it's an associated condition (like certain types of pneumonia I believe with HIV, though for obvious reasons I can't use that disease), that's alright, but his mother should still be able to reasonably and correctly claim that X killed him in her letter to his ex-girlfriend some time after his death.
Various variations of "slowly killing illness", "terminal disease" etc on Google. It gave me WoW quest descriptions, definitions of the term 'terminal illness' and articles about assisted suicide.
Checked the Wikipedia article on Terminal illness hoping for a list or similar, but that doesn't seem to be the case.
Largely irrelevant; medical tech is roughly on par with ours.