Acute sinusitis (symptoms lasting ≤4 weeks) is typically caused by a virus and/or bacteria. Typically viral acute sinusitis precedes bacterial acute sinusitis. A cold or the flu, caused by a virus, causes inflammation of the lining of the nose and sinuses, and results in the symptoms of sinusitis. The swollen sinus lining may block the openings of the sinuses. The result is that mucus can't drain properly and bacteria become trapped in the sinuses. Then the bacteria can begin to grow and start an infection

Typically sinusitis of less than 7 days duration is caused by a virus. You should consult your health care professional if you have had sinusitis for 7 days or more, because the cause may be bacteria that can be treated with antibiotics.

Chronic sinusitis (symptoms lasting >12 weeks) is more complex than acute sinusitis. It may involve infection with bacteria, viruses, or even fungi. Factors such as allergies, nasal polyps (growths in the nasal passages), and a deviated septum (in which the cartilage separating the two nostrils is not straight) may contribute to chronic sinusitis.