Fever in the infant and toddler is one of the most common problems and greatest challenges faced by those caring for them. This article addresses the most common etiologies of fever in these age groups and the appropriate clinical prediction rules for identifying infants and toddlers at lowest risk for serious bacterial infections. (See also Fever Without a Focus and Emergent Management of Pediatric Patients With Fever.)
Neonates (≤28 d) with fever may have few clues on history and physical examination to guide therapy; however, 3% have a serious bacterial infection. Obtaining the pertinent medical history from the mother regarding the pregnancy, delivery, and early neonatal life of the febrile neonate is essential. Typically, infections that occur in the first 7 days of life are secondary to vertical transmission, and those infections occurring after the first 7 days are usually community acquired or hospital acquired.
Definitive identification of a serious bacterial infection requires laboratory investigation; a full sepsis evaluation; and a positive result in blood culture, cerebrospinal fluid (CSF), and/or urine. Bacterial meningitis is more common in the first month of life than at any other time. An estimated 5-10% of neonates with early onset group B streptococcal (GBS) sepsis have concurrent meningitis. 
The general approach to fever in a febrile infant aged 28-60 days includes maintaining a high index of suspicion, because these patients often lack clues on physical examination. The prevalence of a serious bacterial infection in an infant younger than 3 months is approximately 6-10%, most often urinary tract infections (UTIs). Interestingly, infants aged 3 months or younger with a confirmed viral infection are at lower risk for a serious bacterial infection when compared with those in whom a viral infection is not identified  ; although a UTI is still a significant concurrent infection in infants with bronchiolitis.
Children Aged 3 Months to 3 Years
According to guidelines from the Agency of Health Care Policy and Research published in 2012, in infants younger than 3 months with rectal temperatures 38o C or higher, the prevalence of serious bacterial infection reported in studies conducted in North American emergency departments or primary care practices ranged from 4.1-25.1%. 
Historically, children aged 3 months to 3 years with rectal temperatures of 38.5o C or higher had a risk of 2-4% for occult bacteremia.  The leading cause of bloodstream infection was Streptococcus pneumoniae, followed by Haemophilus influenzae type b. With the introduction of effective vaccines for these pathogens, the incidence and epidemiology of childhood bacteremia in the immunologically normal host has changed considerably; only 1 in 200 (0.5%) febrile children are now found to be bacteremic. [5, 6]
The incidence of occult bacteremia in this population now ranges from 0.25-0.7%; moreover, 2 of every 3 blood isolates from these children represent an artifact (contamination) and not a true pathogen. [5, 7, 8] S pneumoniae and Escherichia coli are the most common pathogens, accounting for two thirds of cases. In infants with S pneumoniae, many isolates are strains not covered by the currently available heptavalent conjugate vaccine.
Children with pneumococcal bacteremia may present with acute otitis media, pneumonia, symptoms of sinusitis, meningitis, febrile seizures, cellulitis (including orbital or facial cellulitis), or nonspecific febrile illnesses. E coli bacteremia is most common in children younger than 1 year and is usually associated with urinary tract infection (UTI). Staphylococcus aureus accounts for 15% of bloodstream infections and may be associated with skin, soft tissues, or musculoskeletal infections. Salmonella species, Neisseria meningitides, and S pyogenes account for most of the remaining infections.
As with most patients, the approach to the febrile child aged 3 months to 3 years consists of a targeted medical history, a complete physical examination, and the judicious use of the laboratory tests.
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