Whooping cough continues to spread throughout the U.S., with the collar counties of Illinois hit particularly hard by the contagious bacterial disease that causes violent coughing.

As of Monday, nearly 500 cases had been reported in McHenry, DuPage and Lake counties in Illinois, the  Chicago Tribune reported, but health officials say that cases are likely much higher due to many people who will go undiagnosed. Thirty to 40 new cases around the state are being reported each week.

"This is a vaccine-preventable illness," Debra Quackenbush, spokeswoman for the McHenry County Health Department, told ABCNews.com. "Parents should make sure children are up-to-date on their vaccinations. If you have a cough that seems to linger and it doesn't seem to go away, get it checked out by the doctor just to be sure."

Quackenbush said 172 cases of whooping cough have been reported in McHenry County, Illinois, and she  expects that number to continue to grow. The number is a stark contrast to the nine cases that Quackenbush said the county recorded all of last year.

If a person comes down with whooping cough, doctors will give the patient five days of antibiotics. Health officials encourage the entire family of the patient to go on antibiotics, as well. But more importantly, get vaccinated, they say. The counties have made the tDap vaccine, a shot that prevents diphtheria, tetanus and pertussis, readily available to the public.

Quackenbush said it's hard to say why the collar counties have been hit so hard this year, but it's likely a combination of factors.

"Some people aren't vaccinating their children," said Quackenbush. "They try to skate by without vaccines, but what people don't realize is that, at the beginning of the 20 th century, thousands of people died from this particular illness. These vaccines save thousands of lives."

Nearly a dozen states recently passed laws that require parents to prove that their middle and high-school aged children received a whooping cough vaccination. The law was prompted by a whooping cough outbreak that killed 10 babies and sickened about 9,000 people last year in California.

While people of all ages can come down with whooping cough, even if they've been vaccinated, it's particularly dangerous for newborns' systems because they don't have the immunity or vaccine to fight off the infection. Studies show that about 75 percent of newborns that come down with whooping cough get it from a family member. Eighty-three percent of all deaths from pertussis between 2004 and 2008 were in children less than 3 months old.

Because of the high rate of whooping cough in young infants, the Centers for Disease Control and Prevention recently recommended that pregnant mothers get vaccinated with Tdap, a vaccine to protect against diphtheria, tetanus and pertussis.

"The mother then passes her antibodies to the baby before they are born, and this helps protect the baby against getting whooping cough prior to the time that the infant gets the first three doses of the vaccine," Dr. Jon Abramson, chair of the advisory committee on immunization practices of the CDC, told ABCNews.com last week.

An Associated Press report released last week that found more parents have opted out of school vaccines for their kids. The AP reported that the reasons for declining vaccines varied. Some doubted the need for vaccines, others feared adverse effects and other parents said it's easier to check a box opting out of the shots, rather than go through the necessary paperwork and time.

While many anti-vaccine activists argue that there is not enough sound research for vaccination advocacy, Dr. Mark Sawyer, a professor of pediatrics at University of California at San Diego, said that vaccines are actually the best studied medical intervention out there.

Experts know more about the safety of vaccines than most available surgeries and medications because of the ability to track adverse effects of people who receive various vaccines.

"Those studies are going on all the time scanning for rare vaccine reactions," said Sawyer. "By doing so, we now know that vaccines do not cause autism and they do not cause other serious problems. Although any vaccine can have side effects, in all cases the risk of getting the disease is much greater than the risk of a serious side effect from the vaccine designed to prevent that disease."