The investigators hypothesize that the currently recommended dose of acyclovir is inadequate to produce adequate blood levels to combat herpes simplex infection. The investigators propose to study acyclovir levels in the blood of babies who are placed on acyclovir to treat a suspected HSV infection. This will allow them to determine the appropriate dose in premature infants. This is an unmet public health need because it is likely that the drug behaves differently in premature infants than it does in term infants and older children. Premature babies have more body water and less body tissue. Their kidneys are more immature and do not function as well as full term infants. Premature neonates are also at the greatest risk from herpes infection because they have poorly functioning immature immune systems. Early and appropriate treatment with acyclovir has resulted in improved outcome in term infants.