disorder characterized by persistent debilitating fatigue. There exist two specific criteria that must be met for a diagnosis of CFS: (1) severe fatigue lasting six months or longer and (2) the coexistence of any four of a number of characteristic symptoms, defined as mild fever, sore throat, tender lymph nodes, muscle pain and weakness, joint pain, headache, sleep disorders, confusion, and memory loss. In addition, a diagnosis of CFS requires that the symptoms experienced cannot have predated the onset of fatigue and that all other illnesses or medical conditions capable of giving rise to these symptoms have been ruled out clinically.

CFS was once routinely dismissed as an imagined rather than a specific physical condition, and even today it remains controversial. Indeed, its validity as a distinct disorder is sometimes questioned, since there is considerable overlap with other so-called functional somatic syndromes such as fibromyalgia and Gulf War syndrome. As with these two conditions, some persons initially diagnosed with CFS are later diagnosed with a mental disorder. However, for the majority of people with CFS, the disability and symptoms are physically real and debilitating. A number of theories about the cause of the syndrome have been advanced, but none have been proved. Outside of underlying mental disorders, proposed causes include iron deficiency anemia, autoimmunity to parts of the nervous system, viral infection (e.g., infection with the Epstein-Barr virus), immune system dysfunction, hypoglycemia (low blood sugar), chronic hypotension (low blood pressure), allergies, and altered hormone production. It has been suggested that CFS itself represents a broad category containing subgroups of diseases, all with unique symptoms but all producing the same ultimate affect—fatigue.

There is no cure for CFS. Patients can be treated with drugs to control their most severe symptoms; for example, some patients are treated with antihistamines to control allergy-like symptoms or with sedatives to treat sleep disorders. Many patients respond positively to a range of nondrug therapeutic approaches, including exercise therapy, counseling, and stress reduction. In addition, studies have shown promising responses to cognitive-behavioral therapy, in which patients learn to develop strategies that help them cope with their disease and that improve the quality of their lives. While some persons with CFS become progressively worse, most improve gradually, and a few eventually achieve full recovery.