A condition in which the tissue lining the sinuses (small hollow spaces in the bones around the nose) becomes swollen or inflamed. The sinuses are lined with cells that make mucous. Sinusitis can occur when too much mucous builds up in the sinuses or their openings become blocked. It is usually caused by a viral, bacterial, or fungal infection. Signs and symptoms include headache, facial pain or pressure, stuffy or runny nose, sore throat, fever, cough, fatigue, bad breath, and reduced sense of smell and taste. Having certain conditions, such as allergies, nasal polyps (growths) or other nasal problems, a weak immune system, or certain medical conditions (such as cystic fibrosis), may increase the risk of developing sinusitis. Smoking or being exposed to secondhand smoke may also increase the risk of sinusitis.