Chronic Fatigue Syndrome (CFS), also called Systemic Exertion Intolerance Disorder (SEID) or Myalgic Encephalitis (ME), is marked by severe fatigue, sleep disturbances, cognitive difficulties, pain, and other symptoms that worsen with exertion.

These symptoms persist for at least six months and cannot be explained by any underlying medical condition.

Although the symptoms of CFS are real, the exact cause remains unknown.

If medical evaluations do not identify any disease or injury, or if treatments fail to provide expected improvement, it is likely that the brain plays a role in generating or contributing to the pain or illness. This is called a neuroplastic symptom.