Fever is triggered when immune cells release cytokines (notably interleukin-1), which prompt the hypothalamus to produce prostaglandins and raise the body's temperature set point. This response helps fight infection but also raises metabolic demand and stress on the heart and lungs. While usually beneficial and self-limited in healthy children, fever can be risky for infants with cardiopulmonary or neurologic problems and may precipitate febrile seizures, which require careful evaluation to exclude serious causes like meningitis.

Causes (Etiology)

Causes of fever vary by duration and pattern. Acute fevers (≤14 days) are most often due to infections — commonly viral respiratory or gastrointestinal illnesses — but can also come from bacterial infections like otitis media, pneumonia, or urinary tract infections. Newborns (<28 days) are at higher risk for invasive bacterial infections acquired around birth (for example, group B streptococci, E. coli, Listeria, and herpes simplex).

Recurrent or periodic fevers occur as discrete febrile episodes separated by normal periods and have distinct causes. Chronic fevers (>14 days) or fever of unknown origin (FUO) require a broader evaluation for infection, inflammatory disease, malignancy, or other less common conditions.

Initial Evaluation

A careful history and physical exam guide evaluation. Important details include degree and duration of fever, how temperature was measured, antipyretic use, associated symptoms (poor feeding, lethargy, respiratory or gastrointestinal symptoms), recent exposures, vaccination status, and any predisposing conditions (prematurity, indwelling devices, immunodeficiency). Examination emphasizes overall appearance, vital signs, respiratory status, and focal signs of infection.

Red Flags

Immediate concern is warranted for neonates (<28 days), any child who appears toxic or lethargic, those with respiratory distress, petechial or purpuric rash, or inconsolability. A temperature ≥39°C in children under 36 months carries a higher risk of serious bacterial infection and often prompts more investigation.

Age-Based Testing Approach

Testing is driven by the child’s age and clinical appearance. Neonates require full sepsis evaluation (blood, urine, and CSF studies) and usually hospitalization with empiric IV antibiotics. Infants 1–3 months are triaged based on appearance and labs; ill-appearing infants receive comprehensive testing and empiric treatment, while well-appearing low-risk infants may be managed as outpatients with careful follow-up. Children older than 3 months are tested selectively based on exam and suspected source; well-appearing, fully immunized toddlers with no source have a low risk of bacteremia and often require minimal testing.

Approach to Chronic or Recurrent Fever

For recurrent or chronic fevers, testing is tailored to suspected causes suggested by history and exam. Basic workup for FUO includes complete blood count, inflammatory markers (ESR, CRP), blood cultures, urinalysis and urine culture, chest radiograph, metabolic and liver/renal panels, HIV testing, and tuberculosis screening. Additional targeted tests, imaging, or specialist referral follow based on findings.

Treatment

Treatment targets the underlying cause. Fever alone in an otherwise well child does not always require treatment — antipyretics are used mainly for comfort or to reduce physiologic stress in children with cardiopulmonary or neurologic disease. Common antipyretics are acetaminophen and ibuprofen; aspirin should be avoided in children. Nonmedication measures (tepid baths, cool compresses, light clothing) may relieve discomfort, but avoid cold baths or alcohol rubs.

When to Seek Care

Contact a healthcare provider or seek emergency care for persistent high fever, signs of severe illness (difficulty breathing, poor responsiveness, persistent vomiting, inability to drink), seizures, stiff neck, rash with petechiae or purpura, or new neurologic symptoms. For neonates and very young infants, fever should prompt immediate medical evaluation.