Sinusitis is an inflammation of the nasal mucosa and paranasal sinuses. This inflammation often occurs after an upper respiratory infection or allergy. Children are born with ethmoid and maxillary sinuses, from 5-7 years old the sphenoid sinuses begin to form, from 9 years old the frontal sinuses begin to form, the sinuses gradually complete until 19 years old, so most children have maxillary and ethmoid sinusitis.