Infants or young children who have a fever with no obvious source of infection present a diagnostic dilemma. Health care providers see these patients on a daily basis. As many as 20% of childhood fevers have no apparent cause.  A small but significant number of these patients may have a serious bacterial infection; the risk is greatest among febrile infants and children younger than 36 months, making proper diagnosis and management important. Physical examination and patient history do not always identify patients with occult bacteremia or serious bacterial infection. Serious infections that are not recognized promptly and treated appropriately can cause significant morbidity or mortality.
This article focuses primarily on infants and young children aged 2-36 months and reflects the significant changes in the care of the febrile infant and child over the past 10 years. The article Fever in the Young Infant addresses the diagnosis and treatment of febrile infants younger than 2 months.
Fever is defined as a rectal temperature that exceeds 38°C (100.4°F). Direct the initial evaluation of these patients toward identifying or ruling out serious bacterial infections (SBI), most commonly urinary tract infections. The following questions are important to consider:
What laboratory studies are indicated for various age ranges?
Which patients need in-depth evaluation and treatment?
Which patients need treatment with antibiotics?
Which patients should be hospitalized?
Which patients can be sent home safely and what follow-up is appropriate for them?
Are the diagnosis and treatment modalities for each patient cost-effective?
What is the potential morbidity associated with testing and treatment?
What are the parental (and patient) preferences for testing and treatment?
A great deal of time and effort has been spent on research to help identify the febrile infant and young child with a serious bacterial infection. However, evaluation and treatment of febrile infants and young children vary, despite nationally published treatment guidelines.
Note also, this article primarily addresses children who are completely immunized, and in particular who have received full Hib and PCV7 vaccine series. Unimmunized children are at higher risk for bacteremia, pneumonia, and other SBI's.
Neonates' immature immune systems place them at greater risk of systemic infection. Hematogenous spread of infection is most common in this age group or in patients who are immunocompromised or unimmunized. For these same reasons, infants who have a focal bacterial infection have a greater risk of developing metastatic infection or bacteremia.
The following are among the most common bacterial etiologies of serious bacterial infection in this age group:
Historically, approximately 2.5-3% of highly febrile children younger than 3 years have occult bacteremia, which is typically caused by S pneumoniae.  The advent of conjugate pneumococcal vaccine has resulted in a decrease in pneumococcal occult bacteremia and other disease.  Viral infections are common in the young child as well  ; however, exclude serious bacterial infection prior to assuming a viral etiology for the fever. 
Fever accounts for 10-20% of pediatric visits to health care providers.
Patients with no easily identified source of infection have a small but significant risk of a serious bacterial infection. If not recognized and treated appropriately and promptly, this can cause morbidity or mortality.
There is no racial difference in incidence of fever.
There is no difference in incidence of fever in males vs. females.
This article focuses on the diagnosis and treatment of febrile children aged 2-36 months.
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