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

Post-viral fatigue syndrome

Post-viral fatigue syndrome is a condition or conditions characterized by fatigue following a viral infection. The term is not widely used, but the state of prolonged or severe fatigue after illness is not uncommon...

 (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

Etiology

Etiology is the study of causation, or origination. The word is derived from the Greek , aitiologia, "giving a reason for" ....

 (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

Myalgia

Myalgia means "muscle pain" and is a symptom of many diseases and disorders. The most common causes are the overuse or over-stretching of a muscle or group of muscles. Myalgia without a traumatic history is often due to viral infections...

 and joint pain

Arthralgia

Arthralgia literally means joint pain; it is a symptom of injury, infection, illnesses or an allergic reaction to medication....

; 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.