When I had it, my doctor said that if I was a professional athlete, the standard outcome would be to just consider the current season lost and switch focus to getting well again in time for the next.
I was relatively useless for two weeks and then spent the next four months getting back to my normal energy levels. "Severe" is a question of definitions, I think. I wouldn't consider that kind of illness severe, but it certainly put a dent in things for a while. Properly inconvenient, I'd say. I think this degree is pretty common.
Some develop no noticeable symptoms and many develop mainly annoying ones for a few weeks to a few months. I had the misfortune to develop symptoms from infectious mononucleosis shortly after minor surgery. My GP and surgeon kept giving me antibiotics and sending me back to uni. Finally after several months of fatigue, nausea, and depressed appetite I wandered into campus health jaundiced with a 104ΒΊ F fever and a temporarily enlarged liver. I ended up withdrawing from that semester and retroactively from the previous one. I was ordered into a month of bedrest and half a year of decreased physical activity while recovering. So I guess we could say there's a wide range of severity, partly depending on how soon it's addressed.
Many if not most folks dont' even realize they get it because they catch EBV as a child. In children, it generally causes no symptoms or symptoms so mild that they are dismissed as a normal childhood virus. You know, the sort you miss 2 days of school for and then go on. This is the reason most folks don't know they've had it.
When we get older - teens and adults -, we are more likely to have it develop into mono or other things that realistically make your life difficult for a while.
