"The threshold for eliciting PEM and the frequency and severity of PEM vary with activity, but also illness severity. Learning how to "pace" limits the frequency and severity of PEM. Perfect "pacing" can eliminate PEM---but that doesn't mean the tendency to develop it is gone, or that the patient isn't severely limited. The illness is characterized and defined by activity (all types of activity) intolerance, and the tendency to develop PEM, not the amount that is currently present or its current severity. Illness severity is definitely related to frequency and severity of PEM. I've seen people who feel they are mostly recovered relapse severely by trying to do too much, especially by trying to exercise without respecting the potential for developing PEM and relapse. Severely ill can barely think without triggering PEM."