Chronic fatigue syndrome is a name for a disorder characterised by persistent fatigue that lasts at least six months that cannot be traced to another cause. Its causes and etiology are unknown, but it does tend to present after any viral infection that causes fatigue. It is largely a diagnosis of exclusion as there is no definitive diagnostic test, only a set of agreed upon symptoms that cannot possibly be traced to any other physical or psychological illness.
When CFS is suspected, a physician must first rule out Lyme disease, sleep disorder, clinical depression, alcohol and drug abuse, diabetes mellitus, hypothyroidism, mononucleosis, lupus, multiple sclerosis, hepatitis and several other malignant conditions. All of those conditions are treatable.
CFS cannot be cured, and there is no universally effective treatment. Treatment usually first focusses on having the patient understand the nature of the disease so they can more effectively deal with it. After this, a program of slowly graduated exercise has been shown to improve the condition in about 40% of cases.