WOODSTOCK, Ill. (CBS) — Illinois Department of Public Health officials are seeing the worst whooping cough outbreak in years.

As WBBM Newsradio’s Lisa Fielding reports, a total of 1,019 people throughout the state have contracted the highly contagious bacterial infection, also known as pertussis, according to the Chicago Tribune.

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LISTEN: WBBM Newsradio’s Lisa Fielding reports

The newspaper says half of all those cases are in McHenry, Lake and DuPage counties.

The outbreak began in August and surged in mid-October, after eight Cary Grove High School students in McHenry County were diagnosed.

Since then, 166 McHenry County residents have been diagnosed, in as many as 13 different communities. That includes confirmed cases in at least 29 different schools, the Tribune reported.

McHenry County Health Department spokeswoman Debra Quackenbush emphasized last week that pertussis is not something to take lightly. It is highly contagious.

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She is encouraging parents to get their children vaccinated and to keep them home from school if they are coughing.

Quackenbush says people may not even realize they have pertussis and often times downplay the illness.

“We want people to know that we are getting more and more young children getting sick. Make sure if they are symptomatic, please see your doctor. If not yet, get a vaccination,” she said Dec. 1.

Pertussis is a contagious bacterial infection that causes a range of symptoms. It is easily spread through coughing and sneezing.

Symptoms may appear 5 to 10 days after exposure but can take three weeks to show up. Initial symptoms are similar to those of a common cold but after a week or two can develop into bursts of numerous, rapid coughs with a distinctive “whooping” sound.

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While it can be mild in some cases, it poses a greater risk to infants who are not fully immunized, those with compromised immune systems and older people.