My personal take, just an opinion, is that PEM is not unique to CFS at all -- I think that an exacerbation of symptoms from even mild exercise, complete with extreme fatigue and pain and a whole host of things, is something I've seen personally in plenty of folks with other diseases. I mean, I suppose it depends on how you define PEM, but I'm still really doubtful that you wouldn't find any other disease with a similar reaction to exertion. But I do think that PEM is important to differentiate depression from other sources of fatigue, personally. When people are depressed, activity generally helps boost mood and energy level -- it's hard to get motivated, but once you're exercising a little, you generally feel a bit better. Doctors very much encourage people coping with mild depression to try to get engaged in physical activities as much as they can, at least in my experience. It's been my tactic for low energy/moodiness for most of my life, but when I got sick, one of the early things I noticed was that mild exercise never, never started feeling better. A half hour into any physical activity, even just a walk, and even if I was enjoying myself, I'd feel physically terrible. The next day I would feel utterly exhausted from the effort. I wasn't having trouble with motivation, and I was even enjoying the activity; my body couldn't physically deal with the exertion for long. That, to me, is a very, very clear difference from any depression I've ever known about.