Fever occurs in response to the release of endogenous pyrogenic mediators called cytokines (in particular interleukin-1 [IL-1]). Cytokines stimulate the production of prostaglandins by the hypothalamus; prostaglandins readjust and elevate the temperature set point.

Causes of fever ( see Table: Some Common Causes of Fever in Children Some Common Causes of Fever in Children $\"Some$ ) differ based on whether the fever is acute (≤ 14 days ), acute recurrent or periodic (episodic fever separated by afebrile periods), or chronic (> 14 days), which is more commonly referred to as fever of unknown origin Fever of Unknown Origin (FUO) (FUO). Response to antipyretics and height of the temperature have no direct relationship to the etiology.

History of present illness should note degree and duration of fever, method of measurement, and the dose and frequency of antipyretics (if any). Important associated symptoms that suggest serious illness include poor appetite, irritability, lethargy, and change in crying (eg, duration, character). Associated symptoms that may suggest the cause include vomiting, diarrhea (including presence of blood or mucus), cough, difficulty breathing, favoring of an extremity or joint, and strong or foul-smelling urine. Drug history should be reviewed for indications of drug-induced fever.

For acute fever, testing for infectious causes is directed by the age of the child. Typically, children < 36 months, even those who do not appear very ill and those who have an apparent source of infection (eg, otitis media), require a thorough search to rule out serious bacterial infections (eg, meningitis, sepsis/bacteremia, urinary tract infection). In this age group, early follow-up (by phone and/or outpatient visit) is important for all those managed at home.

Febrile infants between 1 month and 3 months of age who are ill-appearing (eg, abnormal cry, lethargy, rectal temperature > 38.5° C) or who have risk factors for serious bacterial infection (eg, significant congenital anomalies, prematurity, technology dependence, unvaccinated) should be admitted to the hospital and have a WBC count with a manual differential, blood cultures, urinalysis and urine culture (obtained by catheterization, not an external bag), and lumbar puncture with CSF evaluation, including culture and, if available, PCR panel. Inflammatory marker levels (C-reactive protein [CRP], procalcitonin, or both) may be measured to assist with risk assessment. Chest x-ray and a respiratory PCR panel should be done in those with respiratory manifestations, and stool is sent for stool culture or enteric PCR stool test in those with diarrhea. These infants should be given age-specific empiric parenteral antibiotic therapy (eg, ceftriaxone) until serious bacterial infection is ruled out by negative cultures or a specific diagnosis is made.

Well-appearing febrile infants between 29 days and 60 days of age with all of the following are at low risk of serious bacterial infections:

For acute fever, testing for infectious causes is directed by the age of the child. Typically, children < 36 months, even those who do not appear very ill and those who have an apparent source of infection (eg, otitis media), require a thorough search to rule out serious bacterial infections (eg, meningitis, sepsis/bacteremia, urinary tract infection). In this age group, early follow-up (by phone and/or outpatient visit) is important for all those managed at home.

For chronic fever (fever of unknown origin [FUO]), laboratory tests and imaging should be directed toward likely causes of fever based on the patient's age and findings from the history and physical examination. Indiscriminate ordering of laboratory tests is unlikely to be helpful and can be harmful (ie, because of adverse effects of unnecessary confirmatory testing of false positives). The pace of the evaluation is dictated by the appearance of the child. The pace should be rapid if the child is ill-appearing, but can be more deliberate if the child appears well.

Treatment is directed at the underlying disorder.

Most acute fever is caused by viral infections.