More than a hundred middle school students in Illinois called out sick earlier this week reportedly due to an isolated outbreak of the flu.

At least 158 of the 884 students at South Middle School in Arlington Heights were out sick Tuesday, while 185 called out Wednesday with possible cases of the flu, CBS Chicago reported.

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At least 31 of the students were reportedly confirmed to have a certain strain of the flu called influenza A, according to the news station, citing a South Middle School spokesperson.

The massive absence prompted school officials to send a letter to parents.

“Our custodial team is treating some areas of the building more frequently to help prevent the spread of germs,” the letter stated, in part, according to CBS Chicago. “Several of our teachers had students using their school supplies wipes to address personal items and surfaces around them in the classroom/lockers.”

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The school reportedly reached out to the Cook County Health Department for guidance on how to control the spread of the virus.

Neither a spokesperson for the health department nor South Middle School immediately returned Fox News’ request Friday for comment on the outbreak.