Viral upper respiratory tract infection (URTI): This is the most common cause of fever, body aches, and headache in a child of this age group, often due to viruses such as rhinovirus, coronavirus, or influenza.

Influenza: Given the symptoms of fever, body aches, and headache, influenza is a plausible diagnosis, especially during flu season.
Mononucleosis (Mono): Caused by Epstein-Barr virus (EBV), mono can present with fever, headache, and body aches, although it more commonly includes sore throat and lymphadenopathy.
Streptococcal pharyngitis: While primarily causing a sore throat, strep throat can also lead to fever, headache, and general malaise.

Meningitis: Although less common, bacterial meningitis is a life-threatening condition that can present with fever, headache, and body aches, and requires immediate medical attention.
Encephalitis: Inflammation of the brain, usually due to viral infections, can cause fever, headache, and body aches, along with more severe neurological symptoms.
Septicemia or bacteremia: Bacterial infections in the bloodstream can cause systemic symptoms including fever, body aches, and headache, and are medical emergencies.
Lyme disease: In endemic areas, Lyme disease can cause fever, headache, and body aches, and is crucial to diagnose due to its potential for long-term complications if untreated.

Kawasaki disease: An acute febrile illness of childhood that can cause fever, body aches, and headache, along with specific criteria such as rash, conjunctivitis, and lymphadenopathy.
Leptospirosis: A zoonotic infection that can cause a wide range of symptoms including fever, headache, and body aches, more common in individuals exposed to contaminated water or soil.
Brucellosis: A bacterial infection that can be transmitted from animals to humans, causing fever, headache, and body aches, among other symptoms, and is rare in most areas but important to consider in certain exposures.