other factors were occasionally, but non consistently, related to outcome. 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. The incidence rate according to age has two peaks, one at 10–19 and another at 30–39 years (see graph in right margin). 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". In the 1950s, the term "benign myalgic encephalomyelitis" was used in relation to a comparable outbreak at the Royal Free Hospital in London. The descriptions of each outbreak were varied, but included symptoms of malaise, tender lymph nodes, sore throat, pain, and signs of encephalomyelitis. The cause of the condition was not identified, although it appeared to be infectious, and the term "benign myalgic encephalomyelitis" was chosen to reflect the lack of mortality, the severe muscular pains, symptoms suggesting damage to the nervous system, and to the presumed inflammatory nature of the disorder. Björn Sigurðsson disapproved of the name, stating that the illness is rarely benign, does not always cause muscle pain, and is possibly never encephalomyelitic. The syndrome appeared in sporadic as well as epidemic cases. In 1969,