Viral Upper Respiratory Tract Infection (URTI): This is the most common cause of headache and fever in children, often accompanied by other symptoms like cough, runny nose, and sore throat. The high prevalence of URTIs in the pediatric population makes this a likely initial consideration.

Influenza: Especially during flu season, influenza can cause headache, fever, and other systemic symptoms in children. The presence of additional symptoms like body aches and cough would support this diagnosis.
Sinusitis: As a complication of URTI, sinusitis can cause headache (often localized to the face), fever, and purulent nasal discharge. The duration of symptoms and their severity can help differentiate sinusitis from a simple URTI.
Streptococcal Pharyngitis: Although more commonly associated with sore throat, streptococcal pharyngitis can also present with headache and fever. A throat examination and rapid streptococcal test can aid in diagnosis.

Meningitis: A potentially life-threatening condition that requires immediate medical attention. Symptoms can include headache, fever, stiff neck, and altered mental status. Although less common, the severity of meningitis makes it a critical diagnosis not to miss.
Encephalitis: Inflammation of the brain, usually due to viral infections, which can present with headache, fever, confusion, and seizures. Prompt recognition and treatment are crucial to prevent long-term neurological damage.
Brain Abscess: A rare but serious condition that can cause headache, fever, and focal neurological deficits. It often requires neuroimaging for diagnosis and urgent neurosurgical intervention.

Subarachnoid Hemorrhage: Although extremely rare in children, it can cause sudden, severe headache (often described as "the worst headache of my life") and fever might not always be present. Neuroimaging is essential for diagnosis.
Systemic Juvenile Idiopathic Arthritis (sJIA): Can present with fever and headache among other systemic symptoms. It's a diagnosis of exclusion and requires a thorough evaluation including laboratory tests and potentially arthrocentesis.
Leukemia: Certain types of leukemia can present with systemic symptoms including fever and headache due to CNS involvement or metastasis. Prolonged or unexplained symptoms would warrant further investigation including blood tests and possibly a bone marrow biopsy.