Just when it seemed you were getting over your cold, it's gotten worse. Your head is pounding, your face aches, and you've got green goop in your nose. But before you open another can of chicken soup, consider this: You may have sinusitis.

Sinusitis is a bacterial infection in the sinus cavities, and if you have it, you're not alone. "It is the most common chronic disease in the United States," says Dale Rice, M.D., professor and chair of the Department of Otolaryngology-Head and Neck Surgery at USC. Sinusitis hits about 37 million Americans each year.

The sinuses are air-filled pockets within the bones of the face and around the nasal cavity. Eight sinuses are connected to the nose through small openings. What do they do? For one, they secrete mucus that humidifies and warms the air you breathe. Tiny hairs, called cilia, keep mucus from building up by constantly sweeping it out of the sinuses and into the nose.

How does sinusitis happen?

Acute sinusitis occurs when something blocks sinus drainage or prevents the cilia from sweeping properly, Rice explains. Sinusitis is usually preceded by a head cold or allergy or provoked by environmental irritants such as air pollution or cigarette smoke. Very dry air also can cause sinusitis.

The mucus then builds up and keeps bacteria in the sinus cavities, raising the chances of infection.

"The problem is that there are plenty of bacteria in your nose and mouth, and if anything happens to stop that mucus flow, that's when you get an infection," Rice says.

What are its symptoms?

"Sinusitis has symptoms similar to an allergy, with nasal congestion," says Rice. "You also get pressure and pain between and behind the eyes. But you get sneezing and a runny nose with allergies, and you don't with sinusitis."

A cold that lasts longer than 10 days, or seems to get better then suddenly worsens, can be a sign. Pain that afflicts one side of the face or starts when you lean forward also can signal acute sinusitis. Other symptoms include fever, a thick yellow-green nasal discharge, upper teeth that ache, or increasingly severe headaches.

Rice suggests patients call their doctor if over-the-counter medications don't help, or if they feel less alert than usual.

Are there complications?

If you have frequent sinusitis, or the infection lasts for more than three months, you might have chronic sinusitis. Left untreated, sinusitis can lead to serious infections of the eyes and the brain. Sinuses not helped by medication may require surgical drainage.

Some sinus infections require antibiotics, and others eventually dissipate on their own. Check with your doctor for treatment, Rice says. If you are experiencing severe discomfort and can't get to a doctor immediately, he suggests taking a pain reliever and decongestant to suppress swelling.

How is it treated?

"Anything to get moisture into your nose is helpful," Rice says. But don't use decongestant nasal sprays for more than a day, he warns, because the drugs' effectiveness diminishes the longer you use them.

Here are some other suggestions:

• Take a steaming shower or hold a hot, wet towel against your face.
• Use a humidifier or facial steamer.
• Try a saline nasal spray.
• Sip plenty of liquids to thin out nasal discharge.
• Add a few drops of eucalyptus oil to a steamer or hot towel.