Chronic fatigue syndrome (CFS), sometimes also called myalgic encephalomyelitis, is a debilitating condition characterised by unexplained fatigue that lasts for at least 6 months alongside other symptoms that are required for a diagnosis of CFS: headaches, unrefreshing sleep, muscle pain and memory and concentration problems . The prevalence is between 400 and 2,500 adults per 100,000 population [1, 2]. Chronic fatigue (CF) alone, without meeting the full criteria for CFS, is more prevalent but less disabling than CFS . Although the exact pathogenesis of CFS is unknown, research implicates infection, endocrine dysfunction, autonomic nervous system imbalance, depressed mood and altered immunity [1, 2]. Psychosocial factors and physical inactivity have been proposed to be of aetiological significance [1, 2], but there is little research on the relative importance of physical inactivity and psychosocial factors in population samples. Cultural factors are known to influence psychosocial risks for many health conditions; therefore, studies in ethnically diverse populations may yield more information about the relative importance of sociocultural, psychological and behavioural risk factors.
Early reports of CFS from clinic populations seemed to suggest that CFS was more common in women, White majority population and the middle classes [4, 5]. In contrast, some recent population-based research in the United States and the United Kingdom shows that the prevalence of CFS, like many illnesses  is actually higher among people of lower socioeconomic status and minority cultural or ethnic groups [3, 7–11]. Psychosocial influences include social support, which is a protective factor against CFS [12–14], whereas social strain, including gender disadvantage and financial strain, are known risk factors for poor health in general and for CFS in particular [7, 15]. Cultural factors include work-related discrimination, assaults and insults; these are more common amongst some ethnic minorities and are important risk factors for a number of health conditions [16–18]. These stressors have not been investigated in CFS.
Although physical illness may potentially explain greater reports of fatigue , physical illnesses that cause fatigue are among the exclusion criteria for a diagnosis of CFS [1, 2]. For example, the latest international criteria (Centers for Disease Control (CDC), 2006)  allow a CFS diagnosis in the presence of long-standing physical illnesses only if these are stable, treated and do not account for fatigue (for example, hypothyroidism, diabetes mellitus and cancer). In contrast, depression and anxiety can cause fatigue and are common in patients with CFS, but are allowable in meeting the CDC criteria for CFS [2, 21]. Therefore, ethnic variations in the prevalence of anxiety and depression may explain variations of CFS prevalence.
Some studies suggest that physical activity is an effective intervention for CFS . However, the role of physical inactivity and overactivity in causing CFS is uncertain [2, 22–24]. Physical inactivity may play a role in maintaining CFS  and is known to be more common among some cultural and ethnic groups. For example, Indian, Pakistani, Bangladeshi and Chinese men and women were the least likely, in a health survey conducted in England, to be as active as recommended in health guidelines [26, 27]. Therefore, varying levels of physical activity may explain variations in CFS.
This paper presents the findings of a Medical Research Council (UK)-funded study to estimate the population prevalence of CF and CFS in an ethnically diverse sample. In this study, we tested whether there is a consistently higher prevalence of CF and CFS in specific cultural and ethnic groups and whether variations in prevalence can be explained by social adversity (social strain and perceived discrimination), social support, physical inactivity, anxiety and depression.
Our hypothesis was as follows: (1) CF and CFS show differing patterns of prevalence across ethnic groups, and this difference is independent of variations by age, sex and socioeconomic status; and (2) prevalence variations may be explained by variations in psychosocial risk factors and physical inactivity, such that (a) social adversity such as social strain, low social support and perceived discrimination account for a higher prevalence of CFS; (b) physical inactivity accounts for a higher prevalence of CFS; and (c) anxiety and depression account for a higher prevalence of CFS.