(Ivanhoe Newswire) -- In a review released this week in The Lancet, researchers are searching to uncover what causes chronic fatigue syndrome. Many questions are still unanswered.

Chronic fatigue syndrome is not only a difficult ailment to treat -- it's also difficult to define. According to the Centers for Disease Control and Prevention, as many as 500,000 people in the United States have a CFS-like condition.

Experts believe chronic fatigue syndrome, dubbed CFS or CFIDS, is a complex disorder caused by many different factors. CFS causes persistent and unexplained fatigue that significantly hinders daily functioning.

Colleagues from the Radboud University in the Netherlands say different factors can predispose people to CFS, trigger the onset of CFS, and perpetuate the syndrome. Some of the factors known to predispose people to CFS include neuroticism, introversion and inactivity in childhood. Study authors also say genetics may play a part in the development of CFS, as women are more likely to have CFS than men.

There are a number of things that may trigger CFS, including sudden or severe physical or psychological distress as well as an infection caused by the Epstein-Barr virus. Psychological factors appear to be involved in perpetuating complaints among patients with CFS.

Authors say cognitive behavior therapy, which teaches patients to gain control of their symptoms, and exercise therapy are the only two treatments that show success in treating CFS.

In light of the many questions still unanswered, authors of the study say, "More knowledge about the psychoneurobiology of CFS and change processes is needed to improve our understanding of this illness and to allow development of more efficient treatments."

SOURCE: The Lancet, 2006;367:346-355