Sinusitis is an infection of the sinuses,
which are air-filled spaces in the bones of your face around the nose. Like the inside
of your nose, the sinuses are lined with a thin layer of tissue called the mucous
membrane that makes mucus (snot).
When you have a cold or allergies,
the tiny holes that drain the sinuses can get blocked and trap mucus and germs in
the sinuses, which can lead to sinusitis.