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.
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