Chronic fatigue syndrome (CFS) is a disorder of unknown etiology, which probably has an infectious basis. CFS is a state of chronic fatigue, which exists without other explanation, for a year or more, and is accompanied by cognitive difficulties. While the cause of CFS is unknown, the disease probably is an infectious disease with immunological manifestations. CFS is not caused by EBV, one of the viruses that may lead to a state of chronic fatigue. EBV definitely has not been shown to cause CFS, and CFS is not synonymous with chronic EBV or chronic infectious mononucleosis.