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.

See also: Fatigue

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 specific virus has been identified as the cause.

Studies suggest that CFS may be caused by inflammation along the nervous system, and that this inflammation may be some sort of immune response or process.

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.

CFS is diagnosed after the health care provider rules out other possible causes of fatigue, including:

• Drug dependence
• Immune or autoimmune disorders
• Infections
• Muscle or nerve diseases (such as multiple sclerosis)
• Endocrine diseases (such as hypothyroidism)
• Other illnesses (such as heart, kidney, or liver diseases)
• Psychiatric or psychological illnesses, particularly depression
• Tumors