Causes
CFS is only just gaining acceptance as a "real" medical condition. Until recently, many people suffering from CFS symptoms were brushed off as psychiatric cases or were told "it's all in your head."
The exact causes of CFS are unknown, but it appears to be triggered by many different factors. Viral infections, genetic predisposition, environmental toxins, and immune reactions are all considered possible causes of the disorder. The persistent tiredness was once attributed to a virus called the Epstein-Barr virus (this virus is responsible for mononucleosis) but the link remains scientifically unproven and is no longer considered a potential cause.
Recent research has focused on abnormalities in the levels of certain hormones, specifically cortisol (the stress hormone) and the female sex hormones. No one knows exactly what causes the regulation of these hormones to be affected in CFS.
For some people, surgery, a head injury, or other traumas may trigger CFS. Some medications including benzodiazepines, beta-blockers, sedatives, antidepressants, and excessive use of antibiotics can cause fatigue.
It's likely that not one single factor is responsible, but rather a combination of factors. Physical activity and physical or emotional stress seems to make CFS worse. However, further research is needed to figure out its causes and to better understand this puzzling condition.