Chronic fatigue syndrome (CFS) is a condition marked by profound, long-lasting fatigue and multiple other symptoms such as joint pain and muscle pain. A syndrome is not a disease but rather a cluster of symptoms. CFS often starts abruptly but can develop gradually. The cause is unknown. CFS is believed not to be contagious or fatal, though it can be very debilitating.

People with chronic fatigue syndrome experience disruption in one or more aspects of their life, such as work, self-care and family life. The name is misleading because it does not indicate the severity of the disabling symptoms, according to the National Women’s Health Resources Center and CFS support groups.

The syndrome has been controversial, with disagreements over diagnostic criteria, treatment and even the existence of CFS. Patients have sometimes been described disparagingly as having “yuppie flu,” faking a condition or being mentally ill. However, recent research released by the U.S. Centers for Disease Control and Prevention (CDC) supports CFS as having a biological basis. Some research has suggested genetics plays a role.

The U.S. National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) emphasizes that chronic fatigue syndrome is not the same thing as chronic fatigue, which is also a symptom of fibromyalgia, rheumatoid arthritis, lupus and other conditions.

CFS can strike anyone but is most common in young and middle-age women. More than 1 million Americans have CFS, and tens of millions have similar conditions, the U.S. Centers for Disease Control and Prevention (CDC) estimates. It is much more common in women than better-known conditions such as HIV infection or lung cancer, according to the Chronic Fatigue and Immune Dysfunction Syndrome Association of America.

Chronic fatigue syndrome was named in the 1980s, but the condition or similar disorders have been known by other names for centuries. It used to be known as chronic Epstein-Barr because it was once blamed on the Epstein-Barr virus. Current alternate terms for CFS include chronic fatigue and immune dysfunction syndrome (CFIDS) and postviral fatigue syndrome.