Neonatal sepsis is a blood infection that occurs in an infant younger than 90 days old. Early-onset sepsis is seen in the first week of life. Late onset sepsis occurs after 1 week through 3 months of age. Causes: Neonatal sepsis can be caused by bacteria such as Escherichia coli (E coli), Listeria, and some strains of streptococcus. Group B streptococcus (GBS) has been a major cause of neonatal sepsis. However, this problem has become less common because women are screened during pregnancy. The herpes simplex virus (HSV) can also cause a severe infection in a newborn baby. This happens most often when the mother is newly infected.