What causes chronic fatigue syndrome (CFS)?

There is no known cause of CFS, which is also known as myalgic encephalomyelitis or chronic fatigue and immune dysfunction syndrome. Viruses have been associated with the condition, including Epstein-Barr virus, human herpes virus type 6 (HHV-6), enteroviruses and coxsackie B virus. There is evidence of immune system dysfunction, altered blood pressure regulation, endocrine abnormalities and problems with the nervous system in CFS, but it is not known if these are cause or effect. No cause has yet been definitively connected to CFS, but research is ongoing.