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. : 182  Chronic fatigue syndrome is a major cause of school absence. : 183  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 Prevention (CDC) recommends these criteria for diagnosis:
Greatly lowered ability to do activities that were usual before the illness. This drop in activity level occurs along with fatigue and must last six months or longer. Worsening of symptoms after physical or mental activity that would not have caused a problem before the illness. The amount of activity that might aggravate the illness is difficult for a person to predict, and the decline often presents 12 to 48 hours after the activity. The relapse, or crash, may last days, weeks or longer. This is known as post-exertional malaise (PEM).
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.