<qa>  What is the main reason for the CFS?Chronic fatigue syndrome CFS is characterized by unexplained, disabling and long lasting fatigue, as well as pain, impaired memory, sleep difficulties and other symptoms. Epstein-Barr virus EBV infection might precipitate CFS. In this study, 200 adolescents undergoing acute EBV infection will be followed prospectively, and also compared with a group of healthy controls. The aim is twofold: 