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A sinus is a hollow space in the body. There are many types of sinus, but sinusitis affects the paranasal sinuses, the spaces behind the face that lead to the nasal cavity.
The paranasal sinuses have the same mucous membrane lining as the nose.They produce a slimy secretion called mucus. This keeps the nasal passages moist and traps dirt particles and germs.
Sinusitis occurs when mucus builds up and the sinuses become inflamed.
Doctors often refer to sinusitis as rhinosinusitus, because inflammation of the sinuses nearly always occurs with inflammation of the nose known as rhinitis.

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