The chief characteristic of chronic fatigue syndrome (CFS) is a disabling fatigue that can impair normal daily functioning and/or occupational performance. CFS is considered a 3-stage illness that involves predisposing, precipitating, and perpetuating factors. Abnormalities in the central and autonomic nervous systems, possibly linked to a viral trigger and ongoing immune system dysregulation, may play a role in the pathophysiology of the disorder.