Chronic fatigue syndrome (CFS) is a debilitating condition marked by profound, long-lasting fatigue and other symptoms that are not relieved by rest. The cause is unknown. The condition often starts abruptly but can also develop gradually.

CFS can strike anyone but is much more common in women and usually occurs in early or middle adulthood. More than 1 million Americans have CFS, and tens of millions have similar conditions, the Centers for Disease Control and Prevention (CDC) estimates.

The condition is controversial, with disagreement over its definition, diagnosis, treatment and, in some cases, even its existence. According to U.S. government health agencies, CFS can be diagnosed in people who have severe fatigue for six months or more, with other known causes ruled out, and at least four of the following symptoms:

• Impaired concentration or short-term memory
• Sore throat
• Tender lymph nodes
• Muscle pain
• Multiple joint pains without swelling or redness
• Headaches
• Unrefreshing sleep
• Malaise lasting more than 24 hours after exertion

CFS can last months or years, with symptoms appearing and disappearing with no apparent pattern. The symptoms resemble those of many other conditions, such as fibromyalgia, Lyme disease or lupus. Diagnosis typically involves a medical history, physical examination, blood tests and other testing. Usually CFS is a diagnosis of exclusion, meaning that physicians may rule out other conditions before diagnosing it.

There is no known cure or prevention method for CFS. Treatment focuses on relieving symptoms and may include exercise therapy, stress management, biofeedback, cognitive behavioral therapy, energy conservation and medications.