Sinusitis occurs when the mucus membranes that line the sinuses become inflamed. The sinuses are four pairs of air-filled cavities in the area around the eyes and nose, consisting of the frontal sinuses over the eyes, the maxillary sinuses in the cheeks, and the ethmoid and sphenoid sinuses between the eyes.

Sinusitis most often occurs as a complication of a viral respiratory infection, usually the common cold. The cold itself can cause inflammation of the mucus membranes in the sinuses and symptoms of sinusitis, but this usually clears up when the body fights off the cold (usually within two weeks). However, sometimes the inflammation and swelling of the sinuses that occur with a cold can cause mucus to build up and become trapped inside them. When this happens, the trapped mucus becomes a hotbed for bacterial growth. The normally harmless bacteria that reside in the nose and throat can then multiply inside the sinuses and cause a painful infection. Such a bacterial infection inside the sinuses is a major cause of acute sinusitis. When symptoms of sinusitis last longer than two weeks, they are likely caused by a bacterial infection.

Anything that causes mucus to become trapped inside the sinuses or that causes them to become inflamed can cause sinusitis. People with nasal allergies such as those caused by pollen or animal dander are more likely to develop sinusitis because their mucus membranes become inflamed and swell whenever they come into contact with their allergens. Anyone who has chronic nasal problems not caused by allergies is also at increased risk of developing sinusitis. Rarely, a fungal infection can cause sinusitis.

Other factors that can contribute to sinusitis include:

• Deviated nasal septum: If your nasal septum — the wall that separates the nostrils — is crooked, it can restrict the flow through the nostrils and make it more likely for mucus to be trapped in sinuses passages.
• Nasal polyps: Having small growths called polyps inside the nostrils or nasal cavity can narrow the pathway out of the sinuses and make blockage more likely.
• Facial trauma: Having a broken bone in the face may obstruct the sinuses or nostrils and lead to sinusitis.
• Immunodeficiency diseases: If your immune system is not normal due to HIV/AIDS, immunosuppression or some other cause, it can make sinusitis more likely.
• Cystic fibrosis: This genetic disease makes mucus more thick and increases the likelihood of contracting infections all along the respiratory tract, including the sinuses.