Three school buildings in suburban Chicago were closed and students sent home on Wednesday after annual air quality testing of cooling towers found higher-than-normal levels of the bacteria that causes Legionnaires' disease, a school official said.

The buildings included a high school, middle school and one that houses central offices and a separate high school program for District U-46 headquartered in Elgin northwest of Chicago, Chief Executive Tony Sanders said in a statement.

"While risk of exposure to the bacteria was low, we decided, in consultation with the Kane County Health Department, to evacuate staff and students to safe locations as a precaution," the district said in an alert on its website.

Legionnaires' disease is a type of pneumonia caused by inhaling mist infected with the bacteria Legionella. The mist may come from air-conditioning units for large buildings, hot tubs or showers.

The district said all 19 water cooling towers were being properly cleaned and sanitized to remove any trace of the bacteria. It did not say whether any illnesses were reported in connection with the findings.

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The district serves about 40,000 students and is the second largest in Illinois with five high schools, eight middle schools and 40 elementary schools, according to its website.

The building shutdowns follow Legionnaires' outbreaks that killed a dozen people in New York City; 13 in Quincy, Illinois; and sickened scores of inmates at a California prison.

All but one of the dead in Quincy, a city about 240 miles southwest of Chicago, were residents of a veterans home, the oldest and largest such facility in Illinois, officials said.

Legionnaires' disease can lead to severe pneumonia, respiratory failure, kidney failure and septic shock. It cannot be transmitted person-to-person.