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. : 7  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 persons 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 helps 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.
Sleep problems; people may still feel weary after full nights of sleep, or may struggle to stay awake, fall asleep or stay asleep.Additionally, one of the following symptoms must be present:
Problems with thinking and memory (cognitive dysfunction, sometimes described as "brain fog")
While standing or sitting upright; lightheadedness, dizziness, weakness, fainting or vision changes may occur (orthostatic intolerance)

Other common symptoms
Many, but not all people with ME/CFS report:
Muscle pain, joint pain without swelling or redness, and headache
Tender lymph nodes in the neck or armpits
Sore throat
Irritable bowel syndrome
Chills and night sweats
Allergies and sensitivities to foods, odors, chemicals, lights, or noise
Shortness of breath
Irregular heartbeatIncreased sensitivity to sensory stimuli and pain have also been observed in CFS.The CDC recommends that people with symptoms of CFS consult a physician to rule out other illnesses, which may be treatable. Onset
The onset of CFS may be gradual or sudden. When it begins suddenly, it often follows a period of infectious-like symptoms or a known infection, and between 20 and 80% of patients report an onset resembling an infection. : 158  When gradual, the illness may begin over the course of months or years. Studies disagree as to which pattern is more common. : 158 : 181  CFS may also occur after physical trauma such as a car accident or surgery. Physical functioning
The functional capacity of individuals with CFS varies greatly. Some persons with mild CFS lead relatively normal lives with vigilant energy management; persons that are severely ill may be totally bed-ridden and unable to care for themselves.