It's believed to be a brain disorder that sometimes kicks in as a result of viral infection.

The signs are numerous, and vary in intensity, but can combine to leave sufferers feeling wiped out for long periods (way beyond the common recovery time from a virus such as flu).  

It is possible to recover, though many sufferers find it hard to heal, remaining housebound on disability as a result.

A variety of studies by CDC and others have shown that between 1 and 4 million Americans suffer from Chronic Fatigue Syndrome (CFS) or Myalgia Encephalopathy. They are seriously impaired, at least a quarter are unemployed or on disability because of CFS.  

About 75% of patients fit the diagnosis for both Chronic Fatigue Syndrome (CFS) and Fibromyalgia Syndrome (FMS). The cause of CFS and FMS is unknown their course is chronic and require long term management. Both disorders can be diagnosed by a physician only on the basis of symptoms reported by the patient and cannot be confirmed by a single laboratory tests or other objective measure.