When these cavities get infected it is called sinusitis. Sinusitis is usually a self-limiting disease that quickly resolves. In a small percentage of cases, a sinus infection can lead to a more serious infection. The sinuses are a group of cavities that are connected with the inside of the nose. The exact purpose of the sinuses is not known.
Various theories abound.
The most common reason stated is that these cavities help to lighted the overall weight of the skull. The sinuses drain into the nose. If the openings from the sinuses into the nose are blocked, the sinuses are unable to drain. This leads to increased fluid in the sinuses. This fluid serves as an excellent place for bacteria to grow and eventual infection ensues.