Chronic sinusitis inflames the sinuses and nasal passages for as long as three months, and it’s one of the most common chronic conditions in the United States. Its cause is a mystery — making it hard to treat — but a new study suggests it could be the immune system acting too aggressively.

Antibiotics can help some patients, but some get no relief from drugs or even surgery, according to researchers at the Saint Louis University School of Medicine.

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The study suggests an infection in the sinuses isn’t caused by bacteria or fungi. Instead, researchers believe it’s the immune system reacting to harmless microorganisms called commensals, that, in healthy people, cause no problems.

The immune system overreacts to the harmless microorganisms, causing inflammation. The inflammation causes more inflammation, in a mean cycle of stuffiness.

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The study’s author, Rajeev Aurora, professor of molecular microbiology and immunology at Saint Louis University, is looking into whether the findings could help people with chronic sinus infections, a different condition than chronic sinusitis.

“We’re currently looking at patients with chronic infections to see if their microbiome (specific colonies of microorganisms like bacteria that live in various places in the body) is altered in a manner that allows pathogenic bacteria to take infect and colonize the sinus,” Aurora said. “Stay tuned.”

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