Viral upper respiratory tract infection (URTI): This is the most common cause of fever in infants, often accompanied by symptoms like runny nose, cough, and irritability. The age of the baby and the duration of the fever make a viral etiology highly plausible.

Ear infection (Otitis media): Common in this age group, especially if the baby has been exposed to other children or has a history of ear infections. Symptoms might include pulling at the ears, irritability, and difficulty sleeping.
Gastroenteritis: Viral or bacterial infections causing diarrhea and vomiting can lead to fever. The baby might show signs of dehydration, such as fewer wet diapers, dry mouth, and sunken eyes.
Urinary Tract Infection (UTI): Although less common, UTIs can cause fever without other obvious symptoms in infants. Risk factors include uncircumcised males and a family history of UTIs.

Bacterial meningitis: A severe and potentially life-threatening infection that requires immediate medical attention. Symptoms can be non-specific in infants but may include fever, irritability, refusal to feed, and a bulging soft spot on the head.
Sepsis: A systemic infection that can rapidly progress to a life-threatening condition. Infants may show non-specific signs such as fever, lethargy, and poor feeding.
Pneumonia: Can present with fever, cough, and difficulty breathing. In severe cases, infants may show signs of respiratory distress, such as grunting, flaring of the nostrils, and indrawing of the chest.

Kawasaki disease: An autoimmune condition that can cause fever, rash, swelling of hands and feet, red eyes, and lips. It's rare but important to consider due to potential cardiac complications.
Juvenile idiopathic arthritis (JIA): Can present with fever and joint swelling, though it's less common in infants.
Inborn errors of metabolism: Certain metabolic disorders can cause episodic fevers among other symptoms. These are rare and often diagnosed through newborn screening or when other signs are present.