Epstein-Barr virus (EBV) is the cause of heterophile-positive infectious mononucleosis. After primary EBV infection, the virus persists in resting memory B lymphocytes and can be detected in oropharyngeal secretions. Short term (1 month) treatment with oral acyclovir, which inhibits EBV replication, results in loss of virus shedding from the oropharynx, but the virus persists in B cells. The goal of this study is to determine if EBV will no longer persist in B cells in patients treated with long term (20 month) oral valaciclovir (which is metabolized to acyclovir). Blood samples and throat washings will be obtained every three months from individuals who are receiving valaciclovir for treatment of genital herpes simplex virus infection. These samples will be analyzed for EBV DNA to determine if the level of EBV DNA declines or becomes negative with long term antiviral therapy. If the level of EBV DNA becomes undetectable in EBV-seropositive persons during the study, we will ask the patients to return twice a year for five years or if they develop symptoms of mononucleosis, so that EBV DNA in their blood and throat washings can be studied. Knowledge gained from this study should provide important insights into the mechanism of persistence of EBV infection.