Sinusitis is an inflammation, or swelling, of the tissue lining the sinuses. Normally, sinuses are filled with air, but when sinuses become blocked and filled with fluid, germs (bacteria, viruses, and fungi) can grow and cause an infection.

Conditions that can cause sinus blockage include allergic rhinitis, viral infections, nasal polyps (small growths in the lining of the nose), or a deviated septum (a shift in the nasal cavity).

Sinusitis typically takes time to evolve. Usually, the diagnosis is made when you have had upper respiratory symptoms that worsen after five days or persist after ten days.