A cold can sometimes linger, but a stuffy nose and headache that just won't quit after two weeks may be sinusitis, a potentially serious infection.

The American Academy of Allergy, Asthma and Immunology said sinusitis -- which often develops after a cold or bout of allergies -- costs $5.8 billion and causes 18 visits to doctors each year.

"Early on, the symptoms of colds and sinusitis are similar," said Dr. Anju Peters. "But if symptoms are worsening after three to five days, or if they are present for more than 10 days, then sinusitis is the likely culprit."

The condition occurs when the sinuses get blocked, which means that they can't drain. It causes a green or gray nasal discharge, foul tasting post-nasal drip, facial pain and perhaps a light fever.

Sinusitis can last for months, or even years, if not properly treated, the group said. But it can usually be treated with antibiotics or decongestants.