There are cavities called paranasal sinuses in the face and behind the nose, and sinusitis is a condition in which inflammation spreads to these cavities.

It can cause symptoms such as nasal congestion, difficulty breathing, thick (viscous) nasal discharge, headache, and postnasal drip (mucus dripping down the throat).

While there are various contributing factors, it is said that many people develop it after experiencing allergic rhinitis such as a cold or hay fever.