Chronic fatigue syndrome (CFS) is the most common name used to designate a significantly debilitating medical disorder or group of disorders generally defined by persistent fatigue accompanied by other specific symptoms for a minimum of six months, not due to ongoing exertion, not substantially relieved by rest, nor caused by other medical conditions. The disorder may also be referred to as myalgic encephalomyelitis (ME), post-viral fatigue syndrome (PVFS), chronic fatigue immune dysfunction syndrome (CFIDS), or several other terms. Although classified by the World Health Organization under Diseases of the nervous system, the etiology (cause or origin) of CFS is unknown, and multiple psychological and physiological factors may contribute to the development and maintenance of symptoms. There is no diagnostic laboratory test or biomarker for CFS.
Symptoms of CFS include post-exertional malaise; unrefreshing sleep; widespread muscle and joint pain; sore throat; headaches of a type not previously experienced; cognitive difficulties; chronic, often severe, mental and physical exhaustion; and other characteristic symptoms in a previously healthy and active person. Persons with CFS 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.