Health officials are hoping to stop the highly contagious bacterial disease from spreading.
As of Monday, the McHenry County Health Department has identified 41 cases of pertussis, otherwise known as whooping cough. This year's outbreak is much higher than the average 10 cases that are usually reported, and it has so far affected eight area schools.
"At the beginning of October we had two cases at Cary-Grove High School, and that raised a red flag because most of our other cases are here and there, but when you have two cases, it's an indicator that you have an outbreak going on," said the McHenry public health Department's Mary Lou Ludicky.
Hardest hit so far is Cary-Grove High School, where 25 students have fallen ill. According to the health department, most are from the football team, cheerleading squad and band.
District officials tell ABC7 the last case was identified last week. None of the cases have been serious enough to require hospitalization.
Students and parents ABC7 spoke to Monday say the school has done a good job of keeping everyone informed.
"The school told us to wash our hands a lot, stay home if you're sick, and see a doctor if you think you have anything," said Jake Defano, a junior at Cary-Grove.
"Both my kids have appointments this afternoon to make sure that they both have been vaccinated against it, and I think we were told that the shot itself lasts five to 10 years," said parent Kathi Davies.
Whooping cough is highly contagious and easily transmitted through coughing and sneezing.
The first symptoms are similar to those of a common cold. A cough gradually becomes severe and the person will have several bursts of numerous, rapid coughs with a distinctive whooping sound.
"Whooping cough can be very serious, used to be called the 100-day cough, for up to 100 days," Ludicky said. "Our big concern is with infants and babies and with older people who feel that they can't get pertussis."
Vaccinations against the whooping cough run about $50 each. They are available at a lower cost for the uninsured.
To help contain the outbreak the county health department is running a clinic this coming Wednesday at their Crystal Lake facility.