Differences of research participants' cognitive abilities pre- and post-illness onset are naturally variable and are difficult to measure because of a lack of specialized analytical tools that can consistently quantify the specific cognitive difficulties in CFS. The frequency of neuropsychiatric and neuropsychological symptoms is increased in the population of persons with CFS; the cause of this is not understood. Various hypotheses have been advanced to try to explain the relationship between the cognitive symptoms and the illness. Some researchers believe psychiatric causes underlie or contribute to the illness, while other researchers believe the illness causes biochemical and sociological changes in people that produce the symptoms. Cause The cause of CFS is unknown. Both genetic and environmental factors are believed to contribute, but the genetic component is unlikely to be a single gene. Problems with the nervous and immune systems, and energy metabolism, may contribute. CFS is a biological disease, not a psychiatric or psychological condition, and is not caused by deconditioning. However, the biological abnormalities found in research are not sensitive or specific enough for diagnosis. Because the illness often follows a known or apparent viral illness, various infectious agents have been proposed, but a single cause has not been found. Hypothesized infections include mononucleosis, Chlamydophila pneumoniae, human herpesvirus 6, and Lyme disease. Inflammation may be involved. CFS has been observed to develop after mononucleosis and gastroenteritis. Risk factors All ages, ethnic groups, and income levels are susceptible to the illness. The CDC states that Caucasians may be diagnosed more frequently than other races in America, but the illness is at least as prevalent among African Americans and Hispanics. A 2009 meta-analysis showed that African Americans and Native Americans have a higher risk of CFS, though it specifically excluded other more common ethnicities worldwide, and it acknowledged that studies and data were limited. More women than men get CFS. A large 2020 meta-analysis estimated that between 1.5 and 2.0 times more cases are women. The review acknowledged that different case definitions and diagnostic methods within datasets yielded a wide range of prevalence rates. The CDC estimates CFS occurs up to four times more often in women than in men. The illness can occur at any age, but has the highest prevalence in people aged 40 to 60. CFS is less prevalent among children and adolescents than among adults. Blood relatives of those who have CFS appear to be more predisposed,