Chronic fatigue syndrome (CFS), also referred to as myalgic encephalomyelitis (ME), is a medical condition characterized by long-term fatigue and other symptoms that limit a person's ability to carry out ordinary daily activities. Biological, genetic, infectious, and psychological mechanisms have been proposed, but the cause is not understood. Diagnosis is based on a person's symptoms. 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.