Chronic fatigue synrome (CFS) is one of several names given to a poorly understood, variably debilitating disorder of uncertain causation. CFS is thought, based on a 1999 study, to affect approximately 4 per 1,000 adults in the United States. For unknown reasons, CFS occurs more often in women than men, and in people in their 40s and 50s. The illness is estimated to be less prevalent among children and adolescents, but studies are contradictory as to the degree.