Sinusitis is inflammation of the sinuses. It occurs as the result of an infection from a virus, bacteria, or fungus.

Causes
The sinuses are air-filled spaces in the skull. They are located behind the forehead, nasal bones, cheeks, and eyes. 

Healthy sinuses contain no bacteria or other germs. Most of the time, mucus is able to drain out and air is able to flow through the sinuses.

When the sinus openings become blocked or too much mucus builds up, bacteria and other germs can grow more easily.

Sinusitis can occur from one of these conditions:

Small hairs (cilia) in the sinuses fail to properly to move mucus out. This may be due to some medical conditions.

Colds and allergies may cause too much mucus to be made or block the opening of the sinuses.

A deviated nasal septum, nasal bone spur, or nasal polyps may block the opening of the sinuses.

Acute sinusitis is when symptoms are present for 4 weeks or less. It is caused by bacteria growing in the sinuses.

Chronic sinusitis is when swelling and inflammation of the sinuses are present for longer than 3 months. It may be caused by bacteria or a fungus.

The following may increase the risk that an adult or child will develop sinusitis:
Allergic rhinitis or hay fever
Cystic fibrosis
Going to day care
Diseases that prevent the cilia from working properly
Changes in altitude (flying or scuba diving)
Large adenoids
Smoking
Weakened immune system from HIV or chemotherapy