What are sinuses?

The sinuses are cavities, or air-filled pockets, that are near the nasal passage. The sinuses make mucus, which is a fluid that cleans the bacteria and other particles out of the air we breathe.

What is sinusitis?

Sinusitis is an infection of the sinuses near the nose. These infections usually occur after a cold or after an allergic inflammation. There are four types of sinusitis:

  • Acute. Symptoms of this type of infection last less than four weeks and get better with the appropriate treatment.
  • Subacute. This type of infection does not get better with treatment initially, and symptoms last four to eight weeks.
  • Chronic. This type of infection happens with repeated acute infections or with previous infections that were inadequately treated. These symptoms last eight weeks or longer.
  • Recurrent. Three or more episodes of acute sinusitis occur a year.

What Causes Sinusitis?

While mucus helps keep your sinuses clear, it could build up in the sinuses due to colds, allergies, or blockages. This build-up can get in the way of the natural drainage of mucus, which may lead to sinusitis.

Acute sinusitis comes on suddenly, often happening right after an upper respiratory infection, such as a cold. Most acute sinus infections are caused by viruses.

A cold or flu may also cause swelling of the sinus and nasal linings, causing sinus openings to become blocked. This in turn, causes mucus to back up, becoming an ideal place to harbor bacteria growth. Thick, yellow, or discolored mucus is often a sign of infection.

Sinusitis and Allergic Reactions

Some people are sensitive to certain substances, which could cause the body to release histamine. Histamine causes swelling of the sinus and nasal linings. Long-term swelling clogs the sinuses, preventing the tiny hairs, or cilia, in the nasal lining from sweeping away mucus. Allergy symptoms can continue over time, but are less severe than with colds.


A polyp is a sac of swollen tissue, which can be the result of an allergy or infection. It may block the middle meatus, opening where most of your sinuses drain. It may even grow large enough to block your nose.

A deviated septum is when the thin wall inside your nose is pushed to one side, often the result of an injury. This can cause blockage of the middle meatus.

People with chronic nasal problems or allergies are more likely to get acute sinusitis. Sinusitis is also more common with a weakened immune system or another condition that causes the body to make extra mucus.