The sinuses are air-filled cavities (hollow spaces) within the facial bones that are connected to the nose. Sinusitis is characterized by inflammation (swelling and irritation) of the lining of the sinuses. When this happens, the tiny hair-like projections that line the sinus cavities can no longer sweep germs, dust and allergens out to be cleared by the body. Instead, these particles become trapped inside.
This causes many of the symptoms of sinusitis (see below). The inflammation is usually caused by allergies and/or infections (bacterial, viral, and rarely fungal). Anatomic defects and immune system problems may also contribute to sinusitis. Acute or new-onset sinusitis lasts for less than 4 weeks while chronic sinusitis lasts for more than 12 weeks.