Sinusitis is an inflammation of the nasal sinuses. It is usually caused by infection (bacterial or viral), but can also be caused by allergic reactions or other responses to environmental agents.

The sinuses are holes in the skull between the facial bones. There are four large sinuses: two inside the cheekbones (the maxillary sinuses) and two above the eyes (the frontal sinuses).

There are also smaller sinuses (ethmoidal and sphenoidal sinuses) located between the larger ones. The sinuses are...