Vaccines have been highly effective at reducing deadly diseases in the U.S., but that doesn’t mean infections like measles are a thing of the past. In fact, there’s new concern about outbreaks in places where more children are not being vaccinated.

A new study published in the Public Library of Science journal found that opposition to vaccines has been growing in recent years, and measles outbreaks have increased, too. Phoenix has the largest number of unvaccinated kids in the nation, the researchers say, and that makes the area a big target for an outbreak.

To break this down on The Show was one of the study's authors, Melissa Nolan. She is assistant professor of epidemiology at University of South Carolina.

EDITOR'S NOTE: The full map from the study was removed from this story after Baylor University researchers found a discrepancy in the Pennsylvania State Department of Health's data.