Chronic fatigue syndrome (CFS) is a condition of prolonged, severe tiredness or weariness (fatigue) that is not relieved by rest and is not directly caused by other conditions. CFS most often occurs in women ages 30 to 50. The cause of CFS is not known. Some researchers suspect it may be caused by a virus, but no definite viral cause has been found. An immune response may also be involved.

Symptons include:

 Fatigue or tiredness that has never experienced to this extent before, lasting at least six months and not relieved by bed rest
 Fatigue that is severe enough to restrict normal activities by at least 50 percent
 Fatigue lasting more than 24 hours that develops after exercise that would normally be easily tolerated
 Mild fever, sore throat, and headaches
 Lymph node tenderness in the neck or armpit
 Muscle aches and muscle weakness, all over or in more than one location
 Sleep disturbances, such as sleeping too much or difficulty falling asleep or remaining asleep
 Joint pain without joint swelling or redness
 Unusual nervous system symptoms, such as increased sensitivity to light, forgetfulness, irritability, and depression