The symptoms of Chronic Fatigue Syndrome (CFS) are : mental and physical exhaustion, muscle pain and overwhelming tiredness.

The cause of CFS is unknown, and multiple psychological and physiological factors may contribute to the development and maintenance of symptoms. There is no diagnostic laboratory test or biomarker for CFS"

Source Wikipedia

"Despite a vigorous search, scientists have not yet identified what causes CFS. While a single cause for CFS may yet be identified, another possibility is that CFS has multiple causes. Conditions that have been studied to determine if they cause or trigger the development of CFS include infections, immune disorders, stress, trauma, and toxins."

Source : A extensive list of possible contributing factors , US Centers for Disease Control and Prevention.

Notes:

• Recent theories, which asserted that it may be caused by a mouse virus called xenotropic murine leukaemia virus-related virus (XMRV) have now been undermined by two studies which suggest that the positive results of the original XMRV experiments may have been due to sample-contamination (Source : New Scientist 2nd June 2011.)

• Chronic fatigue which lasts for six months or more, but which doesn't meet all the criteria for a diagnosis of CFS is termed Idiopathic Chronic Fatigue (ICF). It's also of unknown origin. See Wikipedia