Community-acquired pneumonia: This is the most likely diagnosis given the symptoms of cough, fever, and the presence of an infiltrate on the chest X-ray. The patient's age and lack of chronic illnesses also support this diagnosis.

Influenza: Given the symptoms of cough, fever, and gastrointestinal symptoms like vomiting and diarrhea, influenza is a plausible diagnosis, especially during flu season.
Gastroenteritis: Although the X-ray shows an infiltrate, the prominent gastrointestinal symptoms could suggest a primary gastrointestinal infection, with the respiratory symptoms being secondary.
Bronchiolitis: Although more common in younger children, bronchiolitis can occur in older children, especially if they have been exposed to respiratory syncytial virus (RSV) or other pathogens.

Pneumocystis jirovecii pneumonia (PCP): Although less likely in an immunocompetent child, PCP can present with similar symptoms and is critical to diagnose early due to its severity and the need for specific treatment.
Tuberculosis (TB): TB can present with chronic cough, fever, and weight loss, and although less likely, it's crucial to consider, especially if there's a history of exposure or travel to endemic areas.
Sepsis: Any child presenting with fever, vomiting, and diarrhea, along with respiratory symptoms, could potentially be at risk for sepsis, especially if there are signs of dehydration or shock.

Mycoplasma pneumonia: This can cause pneumonia that presents similarly to community-acquired pneumonia but might have additional symptoms like headache or rash.
Q fever: A zoonotic infection that can cause pneumonia, although it's rare and usually associated with exposure to animals.
Histoplasmosis: A fungal infection that can cause pneumonia, more common in endemic areas or with exposure to bird or bat droppings.