Chronic fatigue syndrome (CFS) is a chronic, debilitating disorder. It affects the brain and multiple parts of the body. It causes extreme fatigue and is not relieved by bed rest. Physical or mental fatigue often makes the condition worse. Symptoms last at least 6 months and are severe enough to interfere with daily activities.

There is no specific lab test or clinical sign for CFS. No one knows exactly how many people are affected by this illness. The Centers for Disease Control and Prevention estimates that as many as 500,000 people in the United States have a CFS-like condition.

No one knows what causes CFS. For more than a century, doctors have reported seeing illnesses similar to it. In the 1860s, Dr. George Beard named the syndrome "neurasthenia." He thought it was a nervous disorder with weakness and fatigue. More recent experiments have supported his idea that the brain is somehow involved in CFS.

Health experts have suggested some explanations for this baffling illness, including:

  • Various viruses, such as Epstein-Barr virus, enteroviruses, parvovirus B19
  • Complex brain-hormonal imbalance
  • Emotional stress
  • Iron-poor blood—anemia
  • Low blood sugar—hypoglycemia
  • Low blood pressure
  • Environmental allergies or toxins
  • Body-wide yeast infection—candidiasis

Recovery time varies among individuals with CFS. You may recover to the point where you can resume work and other activities. You may continue to experience various or periodic CFS symptoms. CFS typically follows a cyclical course. It alternates between periods of illness and relative well-being. You may also recover completely with time.