CHICAGO (CBS) — Two children in the southwest suburbs have come down with the highly contagious illness pertussis, or whooping cough, and a school has warned other parents about it.

A spokeswoman for the Cook County Department of Public Health said a student at Kolmar Elementary School in Crestwood and the student’s sibling developed whooping cough over the winter break.

The school notified other parents of the situation earlier this week.

Children are supposed to be vaccinated against pertussis along with many other illnesses, but it’s unclear whether the children diagnosed with the disease had received the vaccination.

CCDPH Senior Medical Officer Rachel Rubin said vaccination is the best way to protect yourself and your kids from getting pertussis. DTaP (diphtheria, tetanus, and pertussis) shots have long been required by law in Illinois. Exceptions can only be made for medical and religious reasons.

“Certainly the people that have not been vaccinated are more at risk for getting pertussis, of course; but there are reported cases of people that have been vaccinated that have gotten whooping cough,” she said.

Rubin said the infected children from the Crestwood area likely were contagious at a good time – when they were out of school for winter break, so not exposing dozens of other children to the disease.

Whooping cough is a highly contagious respiratory tract infection that can be serious – even life-threatening – in very young children.

It starts with symptoms similar to a common cold – runny nose, congestion, sneezing, a mild cough, or fever – but can develop into a severe cough after one or two weeks. Pertussis can cause repeated violent coughing resulting in a loss of air, forcing a patient to inhale with a loud “whooping” sound.

Last year, there were 150 confirmed cases of whooping cough in Cook County. There were 190 cases in 2014.