response: I know that the fatigue is not because of exertion or some other medical condition. It's essentially unexplained.
original_response: I am not entirely familiar, but I know it is not due to what usually causes fatigue which is ongoing exertion.
history: Do you know what Chronic fatigue syndrome is?
knowledge: The fatigue is not due to strenuous ongoing exertion, is not much relieved by rest, and is not due to a previous medical condition. Fatigue is a common symptom in many illnesses, but the unexplained fatigue and severity of functional impairment in CFS is comparatively rare.