Chronic fatigue syndrome (CFS) is disabling fatigue lasting more than six months that reduces activity by more than half. CFS is a poorly understood disease involving many body systems. No single cause of CFS has been identified, therefore, it is diagnosed by symptoms and by ruling out other known causes of fatigue by a doctor.

Suggested causes include chronic viral infections, food allergy, adrenal gland dysfunction, and many others. None of these have been convincingly documented in more than a minority of sufferers. In some people there is also difficulty sleeping, swollen lymph nodes, and/or mild fever. When there is muscle soreness, fibromyalgia may be the actual problem. Although CFS is considered a modern diagnosis, it may have existed for centuries under other names, such as “the vapours,” neurasthenia, “effort syndrome” (diagnosed in World War I veterans), hypoglycaemia, and chronic mononucleosis.