Sinusitis, computer illustration. The sinuses are membrane-lined air-filled spaces in the bones of the face. The frontal sinuses are above the eyes. Below these are the many lobed ethmoid sinuses and behind them the sphenoid sinuses. Either side of the nose are the maxillary sinuses. Sinusitis, also known as a sinus infection or rhinosinusitis, is inflammation of the sinuses resulting in symptoms such as thick nasal mucus, a plugged nose, and pain in the face. It can be caused by bacteria, viruses, or fungi. Infections of the sinuses are common and cause inflammation of the sinus lining and excess production of thick mucus (yellow).