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Acute febrile illness

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acute febrile illness clinical approach diagnosis differential guideline

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Acute febrile illness (AFI) is a new fever, commonly lasting less than 7-14 days, with or without an obvious source of infection. It may be a self-limited viral illness, but it can also be an early presentation of sepsis, malaria, dengue, meningitis, pneumonia, urinary infection, enteric fever, rickettsial illness, or other region- and exposure-specific diseases.

Immediate red flags: seek emergency care now

  • Confusion, drowsiness, seizure, severe headache, or neck stiffness
  • Difficulty breathing, chest pain, bluish lips, oxygen saturation low
  • Persistent vomiting, inability to drink, very low urine output
  • Fainting, severe weakness, cold/clammy skin, low blood pressure
  • Petechial or purpuric rash, bleeding, severe abdominal pain
  • High-risk patient: infant, pregnancy, older adult, immunosuppression, cancer therapy, transplant, diabetes, liver/kidney disease
A toxic appearance, agitation, altered mental state, rash, focal neurologic signs, or abnormal vital signs require urgent assessment. Harrison's Principles of Internal Medicine, p. 1033.

Practical clinical approach

  1. Assess severity first: temperature, pulse, blood pressure, respiratory rate, oxygen saturation, mental status, hydration, and evidence of sepsis.
  2. Focused history: duration and pattern of fever; cough, urinary symptoms, diarrhea, headache, rash, joint pain; recent travel; mosquito/tick exposure; animal contact; sick contacts; food/water exposure; medication use; pregnancy and immune status.
  3. Examination: skin and rash, throat/chest, abdomen, neurological signs, joints/soft tissues, and any focal source.
  4. Targeted tests, depending on findings and local epidemiology: CBC, renal/liver tests, urinalysis, blood cultures when bacterial sepsis is suspected, chest imaging if respiratory symptoms, and malaria testing promptly after exposure-risk travel. In a febrile returned traveler, malaria, dengue, and enteric fever are prominent considerations, and malaria needs rapid exclusion. The CDC travel guidance highlights this urgency.
  5. Treatment: fluids and symptom relief may be appropriate in stable patients, but suspected sepsis, meningitis, severe malaria, necrotizing soft-tissue infection, or other critical infection requires immediate hospital treatment. Diagnostic sampling should not substantially delay treatment in an acutely ill patient. Harrison's Principles of Internal Medicine, p. 1033.
Do not self-start antibiotics or antimalarials without evaluation, since the cause and appropriate treatment vary widely.
Recent evidence indicates that the common causes of AFI differ substantially by country and setting, with malaria and typhoid among leading etiologies in many low- and middle-income regions, as summarized in this 2025 systematic review.
If this concerns a specific person, share age, fever duration and maximum temperature, symptoms, location/travel, and any test results.
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