Chronic fatigue syndrome (CFS), also called myalgic encephalomyelitis (ME) or ME/CFS, is a complex, debilitating, long-term medical condition. The causes and mechanisms of the disease are not fully understood. Distinguishing core symptoms are lengthy exacerbations or flare-ups of the illness following ordinary minor physical or mental activity, known as post-exertional malaise (PEM); greatly diminished capacity to accomplish tasks that were routine before the illness; and sleep disturbances. Orthostatic intolerance (difficulty sitting and standing upright) and cognitive dysfunction are also diagnostic. Frequently and variably, other common symptoms occur involving numerous body systems, and chronic pain is common. The unexplained and often incapacitating fatigue in CFS is different from that caused by normal strenuous ongoing exertion, is not significantly relieved by rest, and is not due to a previous medical condition. Diagnosis is based on the person's symptoms because no confirmed diagnostic test is available. Proposed mechanisms include biological, genetic, epigenetic, infectious, and physical or psychological stress affecting the biochemistry of the body. Persons with CFS may recover or improve over time, but some will become severely affected and disabled for an extended period. No therapies or medications are approved to treat the cause of the illness; treatment is aimed at alleviation of symptoms. The CDC recommends pacing (personal activity management) to keep mental and physical activity from making symptoms worse. Limited evidence suggests that rintatolimod, counseling, and personalized activity management help improve some patients' functional abilities. About 1% of primary-care patients have CFS; estimates of incidence vary widely because epidemiological studies define the illness dissimilarly. It has been estimated that 836,000 to 2.5 million Americans and 250,000 to 1,250,000 people in the United Kingdom have CFS. CFS occurs 1.5 to 2 times as often in women as in men. It most commonly affects adults between ages 40 and 60 years; it can occur at other ages, including childhood. Other studies suggest that about 0.5% of children have CFS, and that it is more common in adolescents than in younger children. Chronic fatigue syndrome is a major cause of school absence. CFS reduces health, happiness, productivity, and can also cause socio-emotional disruptions such as loneliness and alienation. However, there is controversy over many aspects of the disorder. Physicians, researchers, and patient advocates promote different names and diagnostic criteria. Results of studies of proposed causes and treatments are often poor or contradictory. Signs and symptoms The United States Centers for Disease Control and