Every now and then you meet somebody whose nose seems all stuffed up, or he complains of pains in the eyes and cheeks, and headaches. When you ask him if he has a cold, he may answer: “No, I have sinus trouble.”

What is a sinus, and why do people get “sinus trouble?” Strictly speaking, a sinus is a space filled with blood or with air. But for most people, the expression “sinus trouble” means an infection of one of the cavities connected with the nose.