Four cases of whooping cough have been reported at Carl Sandburg Middle School in Mundelein, although officials there say the infection is not severe.
Letters were sent to parents last Friday notifying them of the situation and providing general information about the contagious illness. At the time there were three cases and one more has been confirmed since, said Principal Mark Pilut.
"It's minor right now. Hopefully we caught it," he said Tuesday.
The cases of whooping cough -- which is caused by bacteria infecting the mouth, nose and throat and is spread through the air by coughing or sneezing -- were not confined to a single classroom, he added. Sandburg has 595 students.
The school was notified when a parent called asking about homework because two children in the family children would be out awhile.
"We hope we nipped it in the bud because we had diligent parents," he said. "We try to provide as much information without panicking anyone."
The Lake County Health Department reported 91 cases as of Tuesday, compared with 89 cases in 2010.
"So far it's nothing extraordinary," said Victor Plotkin, epidemiologist with the Lake County Health Department. "This year, it's pretty much spread throughout the county."
More cases have been reported in children and adolescents, he added, and the department plans Wednesday to contact health care providers to emphasize preventive strategies.
That also is the case in McHenry County, which reported 99 cases as of Monday, the most since 2004 when 191 cases were documented. Early reports were concentrated in schools but the condition has spread to the general community, said Debra Quackenbush, community information coordinator for the McHenry County Department of Health.
Ages of those infected range from an infant to 37 years old, she said. The department will host a vaccination clinic Wednesday at its health clinic in Crystal Lake.
"We have a lot of adults who don't even think they should be vaccinated," she said.
Symptoms appear five to 21 days after infection, usually only after close contact with an infected person. It can start with cold-like symptoms followed by a cough that gradually can become worse with coughing fits sometimes followed by a high-pitched whooping noise.
Health officials say to cover your mouth when coughing or sneezing, wash your hands and stay home if you are sick.