This is a multicenter trial sponsored by the NIAID Collaborative Antiviral Group. Immunocomprised patients from three patient groups: organ transplant recipients, patients with hematologic malignancies, and patients with solid tumors will be enrolled. Each patient will be entered into the study within 72 hours of lesion development and will be instructed to take one study tablet 5 times a day for 14 days. Clinical and virological assessment will be monitored by weekly followups for 3 weeks and assessment for post-herpetic.