DefinitionChronic fatigue syndrome is a condition of prolonged and severe tiredness or weariness (fatigue) that is not relieved by rest and is not directly caused by other conditions.See also: FatigueAlternative NamesCFS; Fatigue - chronic; Immune dysfunction syndrome; Myalgic encephalomyelitis (ME)Causes, incidence, and risk factorsThe exact cause of chronic fatigue syndrome (CFS) is unknown. Some researchers suspect it may be caused by a virus, such as Epstein-Barr virus or human herpes virus-6 (HHV-6). However, no specific virus has been identified as the cause.Studies suggest that CFS may be caused by inflammation along the nervous system, and that this inflammation may be some sort of immune response or process.Other factors such as age, prior illness, stress, environment, or genetics