Sinus infection, or sinusitis, is an inflammation of the sinuses and nasal passages. A sinus infection can cause a headache or pressure in the eyes, nose, cheek area, or on one side of the head. A person with a sinus infection may also have a cough, a fever, bad breath, and nasal congestion with thick nasal secretions. Sinusitis is categorized as acute (sudden onset) or chronic (long term, the most common type).

Anatomy of the sinuses (also called paranasal sinuses): The human skull contains four major pairs of hollow air-filled cavities called sinuses. These are connected to the space between the nostrils and the nasal passage. Sinuses help insulate the skull, reduce its weight, and allow the voice to resonate within it.

The sinuses contain defenses against foreign viruses and bacteria (germs). If the normal defenses are disrupted, they may allow bacteria normally present in the nasal passages to enter any of the sinuses. Once there, the bacteria may stick to the lining cells and cause a sinus infection.

Acute sinusitis usually lasts less than eight weeks or occurs no more than three times per year with each episode lasting no longer than 10 days. Medications are usually effective against acute sinusitis. Successful treatment counteracts damage done to the mucous lining of the sinuses and surrounding bone of the skull.

Chronic sinusitis lasts longer than eight weeks or occurs more than four times per year with symptoms usually lasting more than 20 days.

The sinuses are covered with a mucus layer and cells that contain little hairs on their surfaces (cilia). These help trap and propel bacteria and pollutants outward.

Acute sinusitis usually follows a viral infection in the upper respiratory tract, but allergy-causing substances (allergens) or pollutants may also trigger acute sinusitis. Viral infection damages the cells of the sinus lining, leading to inflammation. The lining thickens, obstructing the nasal passage.

This passage connects to the sinuses. The obstruction disrupts the process that removes bacteria normally present in the nasal passages, and the bacteria begin to multiply and invade the lining of the sinus. This causes the symptoms of sinus infection. Allergens and pollutants produce a similar effect.

Bacteria that normally cause acute sinusitis are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. These microorganisms, along with Staphylococcus aureus and some anaerobes (bacteria that live without oxygen), are involved in chronic sinusitis.

Fungi are also becoming an increasing cause of chronic sinusitis, especially in people with diseases that weaken the immune system, such as AIDS, leukemia, and diabetes.