You know it's not the flu because no one else is sick, and you know it's not a cold because you had that last week. So what's causing that killer headache and the feeling that your nose is locked in a vise? A quick trip to the doctor's office may confirm that you have sinusitis, a common inflammation of the sinuses that affects 30 million to 40 million Americans every year.

What is Sinusitis?

If you've never given your sinuses much thought, a bout of sinusitis can make you think about nothing else. Sinusitis is used to describe any condition where the sinuses become inflamed. Sinuses are the four pairs of air-filled pockets located around the nose and eyes. They are designed to strengthen your skull, filter the air that comes through your nose, add resonance to your voice and help remove mucus from the body. If they become blocked, mucus can't drain properly and air pressure can build up in the sinuses, resulting in the familiar headache and congestion.

Although "sinusitis" and "sinus infection" are often used interchangeably, an infection is only one of many possible causes. The sinuses can also become inflamed by allergens or other environmental irritants, or if you have structural abnormalities in your nose that interfere with sinus functioning.

Your doctor might suspect sinusitis if you show up complaining of a headache, especially one that gets worse when you lower your head, tenderness around the eyes and nose, and thick greenish-yellow nasal discharge.