The sinuses are openings in the bones around the nose. There are 4 pairs of sinuses connected to the nasal cavity by small openings. Normally, air passes in and out of the sinuses and mucus and fluid drain from the sinuses into the nose.

Acute sinusitis affects approximately 3 out of 1,000 people. Some people never get sinusitis, and others develop sinusitis frequently (especially patients with cystic fibrosis). It usually follows respiratory infections such as colds.

Sinusitis occurs when there is an undrained collection of pus in one or more of the sinuses. Disorders that cause swelling of the membranes of the nose, such as allergic rhinitis or viral respiratory infections, are the most common cause because the swelling prevents fluid from draining out of the sinus normally. A deviated nasal septum or other obstruction of the nose may also trap fluid in a sinus. Occasionally, swimming or immersion of the head in water may allow water and bacteria to enter the sinus, causing irritation and infection. The fluid trapped in the sinus may then become infected with bacteria, viruses, or fungi. Dental infections such as tooth abscess may spread into the sinus and infect it directly. Acute sinusitis is most commonly the result of a bacterial infection.

Chronic sinusitis is much less common than acute sinusitis. When sinusitis recurs frequently, or lasts for a prolonged time, it is classified as chronic.

The pain of sinusitis results from the inflammation itself or from pressure within the sinus from accumulation of undrained fluid.