Sinusitis can be either acute or chronic. Acute sinusitis is often caused by a viral respiratory infection that in some cases leads to a bacterial infection; the condition is short term, resolving once the infection has run its course. Usually, the sinuses do not have bacteria in them, but they are connected to the nose, which does normally harbor bacteria.
To avoid infection, the sinuses need adequate mucus drainage and a functioning immune system. Each sinus cavity has an opening into the nose to allow for drainage of mucus. But anything that causes swelling in the nose may lead to obstruction or blockage of the sinuses, which can cause a feeling of pressure and lead to infection. This includes chronic inflammation in the nasal passages, such as that caused by allergies. In fact, inflammation of the nose and sinuses is believed to be the most important factor causing both acute and chronic sinus problems for patients.
The common cold (viral respiratory illnesses), allergies and factors in the environment are the most common triggers for the development of sinusitis. These can all increase mucus production, change the characteristics of the mucus, and cause swelling in the nose and sinuses. The point at which the common cold ends and a sinus infection begins is not always easy to determine. Other important factors may include tobacco exposure, nasal septal deviation, nasal polyps, dryness and mold sensitivity (rarely). Diseases such as asthma, cystic fibrosis, bronchiectasis, immune deficiencies and immotile cilia syndrome (though rare) are often associated with sinusitis. In many people with sinus problems, the lining of the nose and sinuses is overly sensitive to a variety of factors. Thus there are multiple possible triggers for the development of sinusitis.