Infants are recruited from the maternity wards and neonatal wards of four teaching hospitals in the Netherlands. In total 150 infants, treated with antibiotics because of (a high suspicion of) a perinatal infection during the first week of life, will be recruited. The control group comprises 300 healthy newborns, born in the hospital and needing clinical observation for 24-48 hours for several reasons like maternal comorbidity, low probability of neonatal infection, blood sugar monitoring, meconium containing amniotic fluid, or delivery by caesarean section.