CARY, Ill. (CBS) — The number of cases of whooping cough at a northwest suburban high school has more than doubled.

As WBBM Newsradio’s Lisa Fielding reports, the McHenry County Health Department last week reported nine cases of whooping cough, or pertussis, at Cary-Grove High School.

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Now, 22 students at the school have been diagnosed with the disease. Unconfirmed cases in two other areas of the county are also under investigation, the Chicago Tribune reported.

The common areas of the school are being disinfected throughout the school day and after hours, officials from the school and Crystal Lake-based District 155 told the Tribune.

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Pertussis is usually spread person-to-person through coughing and sneezing. Symptoms can occur up to three weeks after exposure.

Symptoms can develop into bursts of rapid coughs with a distinctive “whooping” sound. The infection is typically mild but can be dangerous for small children who haven’t been properly immunized, officials say.

Whooping cough is a bacterial infection that can be treated with antibiotics.

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Last year, the county only had nine cases in total, but there were 51 in 2009, the Tribune reported.