Most cases of acute sinusitis are caused by viruses and will clear up without treatment within two weeks. Viruses can enter the body through the nasal passages and set off a chain reaction resulting in sinusitis. For example, the nose reacts to an invasion by viruses that cause infections such as the common cold, flu, or measles by producing mucus and sending white blood cells to the lining of the nose, which congest and swell the nasal passages.

When this swelling involves the adjacent mucous membranes of the sinuses, air and mucus are trapped behind the narrowed openings of the sinuses. If the sinus openings become too narrow to permit drainage of the mucus, then bacteria, which normally are present in the respiratory tract, begin to multiply.

Most apparently healthy people harbor bacteria, such as Streptococcus pneumoniae and Haemophilus influenzae, in their upper respiratory tracts with no ill effects until the body's defenses are weakened or drainage from the sinuses is blocked by a cold or other viral infection. The bacteria that may have been living harmlessly in the nose, throat, or sinus area can multiply and cause an acute sinus infection.

Medicines, too, can set off a nasal reaction with accompanying sinusitis. For example, intolerance to aspirin and other related non-steroidal anti-inflammatory medications, such as ibuprofen, can be associated with sinusitis in patients with asthma or nasal polyps (small growths on the mucous membrane lining of the sinuses).

Sometimes, fungal infections can cause acute sinusitis. Although these organisms are abundant in the environment, they usually are harmless to healthy people, indicating that the human body has a natural resistance to them. Fungi, such as Aspergillus and Curvularia, can cause serious illness, in people whose immune systems are not functioning properly. Some people with fungal sinusitis have an allergic-type reaction to the fungi.

Chronic inflammation of the nasal passages (rhinitis) also can lead to sinusitis. Allergic rhinitis or hay fever is the most common cause of chronic sinusitis and is a frequent cause of acute sinusitis. Vasomotor rhinitis, caused by humidity, cold air, alcohol, perfumes, and other environmental conditions, also can result in a sinus infection.

Chronic sinusitis refers to inflammation of the sinuses that continues for weeks, months, or even years. As noted above, allergies are the most common cause of chronic sinusitis. Inhalation of airborne allergens (foreign substances that provoke an allergic reaction), such as dust, mold, and pollen, often set off allergic reactions (allergic rhinitis) that, in turn, may contribute to sinusitis.

People who are allergic to fungi can develop a condition called "allergic fungal sinusitis." As body cells react against these inhaled substances, they release chemical compounds, such as histamine, at the mucosal surface. These chemicals then cause the nasal passages to swell and block drainage from the sinuses, resulting in sinusitis.

Damp weather, especially in northern temperate climates, or pollutants in the air and in buildings also can affect people subject to chronic sinusitis.

Chronic sinusitis can be caused by structural abnormalities of the nose, such as a deviated septum (the bony partition separating the two nasal passages), or by small growths called nasal polyps, both of which can trap mucus in the sinuses.

(The information about sinusitus on our site is taken from a publication prepared by the National Institute of Allergy and Infectious Diseases (NIAID), a component of the National Institutes of Health. NIAID supports research on AIDS, tuberculosis and other infectious diseases as well as allergies and immunology.)