Another review found that children have a better prognosis than adults, with 54–94% having recovered by follow-up, compared to less than 10% of adults returning to pre-illness levels of functioning. Epidemiology
The prevalence rates for CFS/ME vary widely depending on "case definitions and diagnostic methods". Based on the 1994 CDC diagnostic criteria, the global prevalence rate for CFS is 0.89%. In comparison, the prevalence rate for the stricter criteria, such as the 1988 CDC "Holmes" criteria for CFS and the 2003 Canadian criteria for ME (both of which, for example, exclude patients with psychiatric diagnoses), produce an incidence rate of only 0.17%. For an example of how these rates impact a nation: the CDC website notes that  between 836,000 and 2.5 million Americans have ME/CFS, "but most remain undiagnosed".Females are diagnosed about 1.5 to 2.0 times more often with CFS than males. An estimated 0.5% of children have CFS, and more adolescents are affected with the illness than younger children. : 182 The incidence rate according to age has two peaks, one at 10–19 and another at 30–39 years. The effect is seen both in female and in male data, but is more pronounce in females. It was suggested that this occurs because these age groups may be more vulnerable to CFS. The rate of prevalence is highest between ages 40 and 60. History
Myalgic encephalomyelitis
From 1934 onwards, outbreaks of a previously unknown illness began to be recorded by doctors. Initially considered to be occurrences of poliomyelitis, the illness was subsequently referred to as "epidemic neuromyasthenia".
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).