Antiviral prophylaxis involved the administration of antiviral agents such as acyclcovir, gancylcovir, and valacyclovir to patients at risk for certain viral infections or reactivation of viral infection. A variety of antiviral agents have been studied. Antiviral prophylaxis has been evaluated in patients with cancer and those undergoing hematopoietic cell transplantation for prevention of infection from viral agents.