NPR: A 2010 whooping cough outbreak in California was found to be due to parents refusing to vaccinate their children.
[Researchers] compared the location and number of whooping cough, or pertussis, cases in that outbreak with the personal belief exemptions filed by parents who chose not to vaccinate for reasons other than a child's health. (Some children with compromised immune systems aren't able to be vaccinated.)
When the number of people vaccinated drops below 95 percent, a community loses herd immunity to highly contagious germs like pertussis, and babies and other unvaccinated people can get sick. In 2010, 91 percent of kindergarteners were up to date on their shots in California.