This will be a randomized, placebo-controlled, double-blind single-center proof of concept study to evaluate the anti-EBV activity of 4 grams of valomaciclovir (2 grams BID) for 21 days in subjects with infectious mononucleosis documented to be caused by primary EBV infection. Otherwise healthy subjects (≥15 years old) referred to us with a clinical diagnosis of primary infectious mononucleosis will be screened and those with laboratory-confirmed primary EBV infection will be enrolled.