A complex of symptoms of unknown cause, marked by often severe fatigue that is present for at least six months, worsened by exertion, and unimproved by sleep, and that also includes either cognitive impairment or poor tolerance to upright activity. Symptoms also may include chronic pain and weakness, low-grade fever, headache, and swollen lymph glands. Also called chronic fatigue immune dysfunction syndrome, myalgic encephalomyelitis.