What causes sinusitis?

Diagram of sinuses

The sinuses are small air-filled spaces (shown in brown) in the bones behind the forehead, and around the nose and eyes. Image: Lightspring / Shutterstock.com

Sinusitis is most often caused by a virus, and you can get it during or after a cold or flu. The viruses that cause the common cold or flu can enter the sinuses from the nose or throat and cause infection.

Sinusitis can also be caused by bacteria. About 2 out of every 100 people will have a sinus infection caused by bacteria. This can often be as a complication of a viral infection of your sinuses.

If your symptoms started suddenly and are still present more than 10 days after your cold started, or if your symptoms started to improve after your cold and then worsened after about 10 days, you may have a bacterial infection.

Sinusitis can also be caused by hay fever and other allergies.