Acyclovir is a drug used to treat herpes simplex virus (HSV) infections in babies. Appropriate dosing of acyclovir is known for adults and children but acyclovir has not been adequately studied in full-term or premature neonates. HSV is a very serious infection in babies <6 months of age and often results in death or profound mental retardation. HSV leads to profound mental retardation in young infants because the virus attacks the central nervous system.