Differential Diagnosis for 5 Week Old Fever

Single Most Likely Diagnosis

• Viral infection: At 5 weeks old, infants are prone to viral infections due to their immature immune system and exposure to various pathogens. Common viral infections in this age group include respiratory syncytial virus (RSV), influenza, and adenovirus.

Other Likely Diagnoses

• Bacterial infection (e.g., urinary tract infection, pneumonia): Although less common than viral infections, bacterial infections can occur in infants and may present with fever.
• Vaccination reaction: If the infant recently received vaccinations, a mild fever can be a common reaction.
• Teething: Although teething typically starts around 6 months, some infants may begin teething earlier, which can cause fever and irritability.

Do Not Miss Diagnoses

• Sepsis: A life-threatening condition that requires immediate attention, sepsis can present with fever, lethargy, and poor feeding in infants.
• Meningitis: A serious infection that inflames the meninges, meningitis can cause fever, irritability, and bulging fontanelles in infants.
• Congenital infections (e.g., herpes simplex, group B strep): Infants can be born with congenital infections, which can present with fever, rash, and other systemic symptoms.

Rare Diagnoses

• Inborn errors of metabolism: Certain genetic disorders, such as congenital adrenal hyperplasia, can cause fever, vomiting, and other systemic symptoms in infants.
• Kawasaki disease: A rare condition that causes inflammation in the blood vessels, Kawasaki disease can present with fever, rash, and lymphadenopathy in infants.
• Osteomyelitis or septic arthritis: Although rare, these conditions can cause fever, irritability, and swelling in infants, particularly if they have a history of trauma or infection.