WOODSTOCK, Ill. (CBS) — This week has brought more reports of whooping cough in McHenry County, and the numbers have reached the three-figure range.

As WBBM Newsradio’s Lisa Fielding reports, the outbreak began in August and surged in mid-October when the McHenry County Health Department reported nine cases. As of Monday, there were 154 cases of pertussis in McHenry County.

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LISTEN: WBBM Newsradio’s Lisa Fielding reports

“Unfortunately, the numbers are only growing,” said Spokeswoman Debra Quackenbusch.

She says this year will likely surpass the last serious outbreak.

“We had 191 cases back in 2004. At this rate, we’ll like get more cases than that this year,” she said.

Another disturbing trend, she says, is that they are seeing more children between the ages of 6 and 10 getting sick.

Quackenbush says 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.

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.

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.

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The county had nine reported cases in 2010, and 51 in 2009.