Definition of Chronic fatigue syndrome:

Chronic fatigue syndrome is a condition of prolonged and severe tiredness or weariness (fatigue) that is not relieved by rest and is not directly caused by other conditions. To be diagnosed with this condition, your tiredness must be severe enough to decrease your ability to participate in ordinary activities by 50%.

See also: Fatigue

Causes, incidence, and risk factors:

The exact cause of chronic fatigue syndrome (CFS) is unknown. Some researchers suspect it may be caused by a virus, such as Epstein-Barr virus or human herpes virus-6 (HHV-6). However, no distinct viral cause has been identified.

Recent studies have shown that chronic fatigue syndrome may be caused by inflammation of pathways in the nervous system, and that this inflammation may be some sort of immune response or process. CFS may occur when a viral illness is complicated by a problem with the body's immune response. Other factors such as age, prior illness, stress, environment, or genetics may also play a role. CFS most commonly occurs in women ages 30 to 50.

The Centers for Disease Control (CDC) describes CFS as a distinct disorder with specific symptoms and physical signs, based on ruling out other possible causes. The number of persons with CFS is unknown.