It's being called an "isolated outbreak," but at least 158 of the 884 students at South Middle School in Arlington Heights were out sick Tuesday of last week, while 185 called out Wednesday with possible cases of the flu.

At least 31 of the students were reportedly confirmed to have a certain strain of the flu called influenza A, according to CBS-2 Chicago, citing a South Middle School spokesperson.

From Fox News:

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.”

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