Normal volunteers who are not taking any antiviral medicines and patients enrolled in NIH's protocol no. 97-I-0168 (Evaluation of Valaciclovir for Prevention of Herpes Simplex Virus Transmission) or Glaxo-Wellcome protocol HS2AB 3009 at collaborating centers may be eligible for this study. Patients in the multi-center study must be about to start valaciclovir therapy for at least 1 year. All candidates must be 18 years of age or older.