The febrile infant is a common clinical problem that accounts for a large number of ambulatory care visits. Young febrile infants (ages 0-3 months) often present with nonspecific symptoms and it is difficult to distinguish between infants with a viral syndrome and those with early serious bacterial illness (e.g., meningitis, bacteremia, urinary tract infection (UTI), and pneumonia). The definitions of serious bacterial illness (SBI) vary across published...