Sinuses are hollow cavities found in the skull that contain a layer of mucus and cells that help trap and expel bacteria and other pollutants. Sinuses are found behind and between the eyes, behind the cheekbones, and in the forehead. When the lining of the sinuses become inflamed, it is called sinusitis, or a sinus infection.

Sinusitis occurs when the cells in the sinus lining become damaged, either due to a viral respiratory illness or exposure to allergens, fungi, or pollutants.. The cells are no longer able to effectively fight the bacteria present in the nasal passages.

Symptoms of sinusitis are pain or pressure in the eyes, cheeks and head, as well as cough and congestion. People with a sinus infection may have a fever, bad breath, and persistent nasal secretions (runny nose.)  Acute sinusitis usually lasts less than eight weeks. Sinusitis lasting more than eight weeks, or occurring more than four times per year with each occurrence lasting more than 20 days, is called chronic sinusitis.