A recent increase in vaccine exemptions correlates with an increase in disease.

Vaccines are key to promoting herd immunity, which in turn keeps individuals with weaker immune systems safer. But, according to a study published in weekly medical journal PLOS Medicine, the number of "philosophical belief"-based nonmedical exemptions have increased since 2009, which correlates with an increase in preventable diseases such as measles and mumps. 

 Click to the next slide to see which cities have recently seen increases in nonmedical exemptions to vaccines.