Fatigue is a common symptom in many illnesses, but CFS is a multi-systemic disease and is relatively rare by comparison. Symptoms of CFS include post-exertional malaise; unrefreshing sleep; widespread muscle and joint pain; cognitive difficulties; chronic, often severe, mental and physical exhaustion; and other characteristic symptoms in a previously healthy and active person. CFS patients may report additional symptoms including muscle weakness, increased sensitivity to light, sounds and smells, orthostatic intolerance, digestive disturbances, depression, and cardiac and respiratory problems. It is unclear if these symptoms represent co-morbid conditions or are produced by an underlying etiology of CFS. All diagnostic criteria require that the symptoms must not be caused by other medical