The upper respiratory tract, which conducts air from the outside down toward the lungs, begins with the mouth and nose, and includes the maxillary, ethmoidal, frontal, and sphenoidal sinuses. The same mucous membrane that lines the inside of your nose also extends into the sinuses, and the mucous produced there drains into your nasal cavity. Sinusitis is an inflammation of the lining of the sinuses, but the nasal passages are usually also involved. Usually, sinusitis develops after a viral upper respiratory infection, like a common cold. The virus causes the lining of your sinuses to become inflamed, so that the process that removes the bacteria normally found in your nasal passages is disrupted, leading to infection. While a case of acute sinusitis typically does not last longer than four weeks, chronic sinusitis lasts longer than three months [source: American Academy of Allergy Asthma & Immunology]. You are more at risk of developing chronic or recurrent acute sinusitis if you have allergies, like hay fever, or if you have any structural defect in the anatomy of your nasal passages or sinuses.
The symptoms of sinusitis depend on whether the infection is acute or chronic, and which sinuses are affected. Symptoms of acute sinusitis might include a stuffed nose with discharge, fever and headache, or pressure around the eyes, temples, or upper teeth. The pain typically changes with the position of your head. Chronic sinusitis often causes a persistent low-grade headache or toothache, chronic post-nasal drip, sore throat, and bad breath.
Doctors can usually diagnose acute sinusitis based on your signs and symptoms. It is important that the doctor determine whether you are suffering from a common cold caused by a virus, or whether you have bacterial sinusitis, in order to decide whether you should take antibiotics. Nasal sprays can also help relieve the inflammation of the nasal passages, though sometimes people overuse them and experience "rebound," where they need to use the spray more and more frequently to be able to breathe comfortably.