To elucidate relation between virus infection and fatigue degree to estimate the virus infection as a fatigue degree index because immune strength is decreased when fatigued and the virus infection is listed as one reason of the immune strength decrease, and to provide a method for screening anti-fatigue substances.

The relation of fatigue degree caused by daily life or disease is elucidated and simply and quantitatively estimated by collecting the body fluid of a test subject and measuring human herpes viruses in the body fluid. Further, the anti-fatigue substances are screened by measuring the anti-fatigue force of anti-fatigue substances or anti-fatigue foods in living body.

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