Chronic Fatigue Syndrome most commonly occurs
in women ages 30 to 50.

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 chronic fatigue syndrome, the tiredness must be severe enough to decrease ability to participate in ordinary activities by 50%. 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 autoimmune process. CFS may occur when a viral illness is complicated by an inadequate or dysfunctional immune response. Other factors such as age, prior illness, stress, environment, or genetic disposition may also play a role.