Viral upper respiratory tract infection: This is the most common cause of fever in infants, and the lack of specific symptoms (no Sx) makes it a likely diagnosis. The fact that the fever is not responding to Tylenol does not rule out a viral etiology.

Urinary tract infection (UTI): Although less common, UTIs can present with nonspecific symptoms such as fever alone in infants. The absence of symptoms does not exclude this diagnosis.
Otitis media: Ear infections are common in this age group and can cause fever without other obvious symptoms, especially if the infant is not pulling on their ears or showing signs of discomfort.
Gastroenteritis: Although gastrointestinal symptoms might be expected, some infants with gastroenteritis may present with fever alone initially.

Bacterial meningitis: Although rare, bacterial meningitis is a life-threatening condition that requires prompt diagnosis and treatment. Fever can be an initial presentation, and the absence of other symptoms does not rule out this diagnosis.
Sepsis: Neonatal sepsis can present with nonspecific symptoms, including fever, and is a critical diagnosis not to miss due to its high mortality rate if untreated.
Pneumonia: Can present with fever alone, especially in young infants, and is a critical diagnosis to consider due to the potential for rapid progression.

Congenital infections (e.g., congenital syphilis, toxoplasmosis): These are less common but should be considered in the differential diagnosis, especially if the infant has risk factors or if other diagnoses have been ruled out.
Inborn errors of metabolism: Certain metabolic disorders can present with episodes of fever without other specific symptoms, although this would be unusual as a first presentation at 5 months.
Kawasaki disease: Although more commonly associated with older infants and toddlers, Kawasaki disease can rarely present in younger infants and is a diagnosis to consider if fever persists without another clear cause.