Acyclovir, Antiviral Agents, Aspartate Aminotransferases, Diagnosis, Differential, Diagnostic Imaging, Electroencephalography, Herpes Simplex, Herpesvirus 1, Human, Herpesvirus 2, Human, Humans, Infant, Infant, Newborn, Infant, Premature, Diseases, Infusions, Parenteral, Proportional Hazards Models, Prospective Studies, Treatment Outcome