Chronic fatigue syndrome (CFS) is a complicated disorder characterized by extreme fatigue that can’t be explained by any underlying medical condition. The fatigue may worsen with physical or mental activity, but doesn’t improve with rest.
Chronic fatigue syndrome has also been called myalgic encephalomyelitis (ME) and, more recently, systemic exertion intolerance disease (SEID). Although CFS/ME and SEID share the same major symptom of chronic fatigue, there is variation between the definitions of these disorders. The symptom of chronic fatigue also may arise from more than one underlying condition.
The cause of chronic fatigue syndrome is unknown, although there are many theories — ranging from viral infections to psychological stress. Some experts believe chronic fatigue syndrome might be triggered by a combination of factors.