This study will examine the effects of long-term antiviral therapy with valaciclovir (Valtrex) on Epstein-Barr virus infection. This virus infects more than 95 percent of people in the United States. Most are infected in childhood, have no symptoms, and are unaware of their infection. People infected as adolescents or adults may develop infectious mononucleosis, which usually resolves completely. Once infected, most people shed the virus from their throat occasionally, and all carry the virus in their white blood cells for life. This study will determine whether the amount of virus in the blood declines or disappears with long-term valaciclovir treatment.