Chronic fatigue syndrome (CFS), also referred to as myalgic encephalomyelitis (ME), is a medical condition characterized by long-term fatigue and other persistent symptoms that limit a person's ability to carry out ordinary daily activities. Other symptoms may include difficulty with thinking or memory, difficulty with sitting or standing, muscle pain, headache, tender lymph nodes in the neck or armpits, recurring sore throat, digestive issues, night sweats, or sensitivities to foods, chemicals, or noise. Symptoms may develop gradually or suddenly, and are often worsened by normal physical or mental activity.

While the cause is not understood, proposed mechanisms include biological, genetic, infectious, and psychological. Diagnosis is based on a person's symptoms because there is no confirmed diagnostic test. The fatigue in CFS 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 found in CFS is comparatively rare in these other illnesses.

There is no cure; treatment is directed toward improvement in symptoms. No medications or procedures have been approved in the United States. Evidence suggests that cognitive behavioral therapy (CBT) and a gradual increase in activity suited to individual capacity can be beneficial in some cases.

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