Sinusitis is most often the result of a viral infection that causes the mucous membrane lining the inside of the nose and the sinuses to become inflamed.

• The mucous membrane swells when it becomes inflamed, blocking the drainage of fluid from the sinuses into the nose and throat.
• Mucus and fluid build up inside the sinuses, causing pressure and pain.
• Bacteria are more likely to grow in sinuses that are unable to drain properly. Bacterial infection in the sinuses often causes more inflammation and pain.

While colds usually trigger this process, any factor that causes the mucous membrane to become inflamed may lead to sinusitis. Many people with nasal allergies (allergic rhinitis), for instance, are likely to have recurring or long-term (chronic) sinus infections. Nasal polyps, foreign objects (usually in children), structural problems in the nose such as a deviated septum, and other conditions can also block the nasal passages, increasing the risk of developing sinusitis.

Fungal infections may also cause sinusitis. This is especially true in people with impaired immune systems. Fungal sinusitis tends to be chronic and more difficult to treat than bacterial sinusitis.