Omphalitis is a polymicrobial infection of the umbilical stump. The mean age of onset is 5-9 days in full-term infants and 3-5 days in preterm infants. It is typically caused by a mix of aerobic and anaerobic organisms. The most commonly identified organisms include Staphylococcus aureus, group A Streptococcus, Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis. Risk factors for omphalitis include prior umbilical instrumentation, low birth weight, septic delivery, and prolonged rupture of membranes. Omphalitis occasionally develops in infants with an underlying immunodeficiency.