According to the Centers for Disease Control and Prevention, Chronic Fatigue Syndrome (CFS) is described as follows:
Chronic fatigue syndrome, or CFS, is a devastating and complex disorder. People with CFS have overwhelming fatigue and a host of other symptoms that are not improved by bed rest and that can get worse after physical activity or mental exertion. They often function at a substantially lower level of activity than they were capable of before they became ill.
Besides severe fatigue, other symptoms include muscle pain, impaired memory or mental concentration, insomnia, and post-exertion malaise lasting more than 24 hours. In some cases, CFS can persist for years.