BOSTON (CBS) – A number of children on Cape Cod have been diagnosed with whooping cough. It’s a serious illness that can be prevented with a vaccine.

These new cases are all being reported in Falmouth. Just this past week, the I-Team reported on the high number of kids on the Cape who are not getting vaccinated.

Massachusetts is one of just 18 states that allow a parent to exempt their children from vaccines on philosophical grounds.

Data examined by the I-Team showed the number of exemptions has quadrupled over the past 25 years. Public health officials tell WBZ that once those exemption rates start to climb, so does the risk of an outbreak.

All of the whooping cough patients in this outbreak, 15 in total, go to Falmouth High School. Also known as pertussis, this is a bacterial infection that spreads easily and is most dangerous in younger patients.

The I-Team obtained new data from the Public Health Department and found the number of Cape Cod kindergarten students exempted from immunizations is almost five percent and up from last year.

“Once vaccination rates go down, the likelihood of an outbreak increases and this has happened both in Minnesota and California,” says Dr. Sharon Daley, Chief of Pediatrics at Cape Cod Hospital.

“Pertussis, otherwise known as whooping cough, in California there have been over 1,000 cases in the past decade, and over 10 deaths, mainly in young infants,” Dr. Daley says.

No one knows for sure why exemption rates are so high on the Cape. But in this case, a school official tells WBZ that all the students had been immunized.

“None of these vaccines are 100 percent effective and there is some waning of effectiveness over time,” said Dr. Daley.

Public health officials maintain vaccines are the best protection from a host of diseases, adding any research linking the shots to autism has been thrown out.

Dr. Daley says the risks of going unvaccinated are well known.

“These kids who are unimmunized are at risk for serious diseases and spreading it to others,” Dr. Daley says.

The I-Team also found exemption rates in Hampshire and Berkshire counties are similar to Cape Cod, and much higher than the state average.