Notably, late-onset (infection occurring at > 7 days of age) group B streptococcal infection is not affected by chemoprophylaxis during labor, and other serious bacterial illnesses such as UTI (most commonly caused by Escherichia coli) and occasional cases of Salmonella and Staphylococcus aureus bacteremia continue to be important causes of fever without apparent source on physical examination in infants < 3 months.