The problem is that fatigue is not a simple thing, and its not precise. Its like saying pain, or being sick, or depression. Its a general term. We do know that fatigue perception is in part based upon acid sensors in the blood vessels. The Lights did research on this. So if we push and our lactate goes up we will feel increased fatigue. This is a surrogate for the actual problem, the energy problems that lead to lactate in the first place. Most cases in the literature are from oxygen supply issues. This is what happens during exercise, and from some kinds of vascular problems and poisoning. Its not clear that our problem is primarily about oxygen, its more about mitochondrial regulation.I'm sorry about that, I would appear to be unable to communicate what seems to me to be a very simple thing.
So everyone can feel fatigue, but the closest sensation to our problem is when hitting the wall during intense exercise, that point when you cannot produce more energy but will shortly get your second wind as the body switches primary energy sources. We live at that wall, and for severe patients and worse I think its 100% of us live there 100% of the time, though that is speculation. I would love a formal test showing this, or disproving it.
We also know that fatigue can be produced from cytokines and other immune factors. That may play a role too.