My dad was diagnosed with AF, which seems to have been caused by viral damage to his heart, in 1984 - he'd have been 43 at the time. It was severe, and hospitalised him for 9 months at no notice. *He* initially thought that he had flu !
Thanks to a superb NHS hospital, and a very loyal and caring employer, he returned to work about a year later, and continued to a normal retirement age - apart from a couple of patches when it got messy again and put him into hospital for shorter periods. My mum has staved off the occasional nervous breakdown as a result. He remains on a cocktail of drugs that I'm not qualified to talk about in detail, but have needed careful monitoring and adjustment and have occasionally had undesirable side effects (fainting at one point, and at another point turning his skin a rather dark shade incompatible with his caucasian ancestry.)
He's not a pilot, and never had to hold a professional medical of any description, but had in his youth been a very fit athlete (100 mile cycle road races anybody?). In all likelihood that residual fitness saved his life in 1984/5, but I doubt he could have passed a class 2, let alone a class 1, at any point in the last 28 years. On the other hand, he remains an active 70 year old now.
So with good medical treatment, at its most severe it appears to be treatable, but not curable. I think it certainly has potential to be career ending for a professional pilot, based upon my dad's experience - but clearly from earlier posts not necessarily and that must depend a lot on the detail of the individual's problem.
Dad was ordered to stop smoking (he had a 3 cigars a week habit!, but apparently even that was too much) and that he could continue drinking, but never afford to get anywhere close to drunk. He's stuck to that advice, and a regular low level exercise regime - and I have every expectation of his coming to my retirement party in 20++ years time.
In his late 60s he was diagnosed with mild diabetes, whether that has any connection, I don't know, but doubt it - I only mention it for completeness.