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.