Researchers think they now know what contributed to the biggest outbreak of whooping cough in California in 47 years.  A new study ties it to a cluster of people who refused to have their children vaccinated for the disease.

"When you have conscientious objectors to immunizations, the rates of those diseases go up in the community," said Dr. Ellen Rome, who did not take part in the study but is a pediatrician at Cleveland Clinic Children's.

Johns Hopkins researchers looked at the number of people who refused the whooping cough vaccine, otherwise known as non-medical exemptions.  Their children entered kindergarten between 2005 and 2010- the year the outbreak occurred.

They found a link between geographic areas, with a large number of people who refused the vaccine, and whooping cough.  Researchers say with diseases like measles and whooping cough, which is also known as pertussis, about 95% of the population must be immunized to prevent outbreaks.

They conclude that large numbers of unvaccinated or under-vaccinated people can lead to whooping cough outbreaks, putting vulnerable populations like infants at increased risk.   Rome agrees.       

"Non-vaccinated who are exposed to whooping cough or pertussis are at risk of dying," she said. "If adults are non-vaccinated we may just get a cold or a bad cough, but for kids that can be fatal."   

Read more on this study in the journal Pediatrics.