This is an infection of the lining of one or more of the air sinuses that develop around the nasal cavity in older children (especially the maxillary sinuses). Sinusitis is usually caused by a bacterial infection, which complicates a common cold. The common cold virus causes swelling of the mucus membranes lining the sinuses. As a result, mucous in the sinuses cannot drain normally and secondary bacterial infection starts a few days after the signs of the common cold.

Acute sinusitis is uncommon in preschool children as their facial sinuses are not yet fully formed. Sinusitis is usually acute but can become chronic. Less commonly sinusitis may complicate allergy.