Hailey Davis, of far northwest suburban Huntley, went to the doctor's office hoping to get medication to stop her cough and runny nose.

She learned she didn't have the flu or a common cold. After a few doctor visits in late September, Davis, 18, was diagnosed with pertussis, or whooping cough. Her harsh, dry cough only worsened, with fits that would last for nearly five minutes. It took about six weeks for the symptoms to clear up.

"I would have such a bad coughing fit, I would struggle to breathe," the McHenry County College freshman said. "I don't know how I got it."

Infectious disease specialists would say that's no big mystery. Cases of the highly contagious bacterial infection have grown throughout the Chicago area, with some counties reporting their highest numbers in nearly five years. Statewide, 1,100 people have contracted pertussis in 2011 through November, health officials said. There were 1,057 cases all of last year.

Public health officials fear the worst is yet to come as students prepare for winter break and families gear up for holiday travel.

"There is going to have to be a change in strategy at some point," said Debra Quackenbush, spokeswoman for the McHenry County Health Department. "I don't see an end in sight. The vaccine is out there. The education is out there. Now people need to do something with that."

There have been more than 650 total confirmed cases of whooping cough in McHenry, DuPage and Lake counties, according to health officials. Confirmed cases have spiked over the last few weeks.

So far this year, Chicago has been spared the severity of the suburban outbreaks, said Efrat Stein, spokeswoman for the Chicago Department of Public Health. The city had 39 confirmed cases through October, the most recent data available. Chicago saw 70 cases over the same period last year.

"Although we are not seeing an increase, we are closely monitoring reported cases as well as what's taking place in the suburbs," said Dr. Julie Morita, deputy commissioner with the city's health department. "We encourage everyone to make sure they are up to date with their vaccines."

In McHenry County, what started with eight cases among students at Cary-Grove High School in Cary has grown into an outbreak with more than 200 cases — a county record — affecting as many as 16 communities and at least 33 schools, health officials said.

"The majority of cases are definitely at the schools," Quackenbush said. "You can only wonder where the tipping point is going be on whether or not people are going to take this seriously."

There have been 138 confirmed cases in Lake County, prompting health officials last week to issue an alert urging residents to be aware of the symptoms, including coughing, spasms and vomiting.

The county expects to surpass its previous record of 164, which is its highest number in five years, according to Dr. Victor Plotkin, a Lake County Health Department epidemiologist.

Unlike in McHenry County, Lake County's cases are not as clustered around specific schools or locations but appear to be more evenly distributed, he said.

In suburban Cook County, numbers have increased by 70 in the last week to 188, said Amy Poore, health department spokeswoman. Each month has seen a slightly higher number of cases than the same period last year, "but we haven't seen a spike in cases that would indicate anything out of the ordinary," she said.

Cases in Will and Kane counties have remained steady at 25 and 38, respectively. DuPage County had 215 confirmed cases as of Thursday.

Experts have theorized that the general increase may relate to genetic changes in the bacterium that causes the disease, making it more potent.

Also, "the newer vaccine doesn't last as long as the older one," said Shawn Cesario, communicable diseases nurse for Lake County. "People become susceptible after three or four years."

Another cause may be a greater awareness of the disease, leading to a higher number of reports. Also, in general, the disease has a cyclical nature, so every few years there's a measurable outbreak, Plotkin said.

The illness typically begins with symptoms similar to a common cold but with significant coughing. Prompt treatment by doctors is key because over-the-counter medicine is ineffective, officials said. The dry cough can last up to 10 weeks.

According to the Centers for Disease Control and Prevention, babies are among those most severely affected. More than half of all children younger than 1 who contract the disease are hospitalized.

In 2010, 27,550 cases of pertussis, with 27 deaths, were reported in the U.S., according to the CDC. Twenty-five of the deaths were children under 1.

Tribune reporters Jennifer Delgado and Sue Ter Maat contributed.

lsynett@tribune.com