Whether hipsters or homeschoolers, parents who don't vaccinate are free riders. Their children benefit from herd immunity without assuming the very small risk of adverse reaction to vaccination. It is a game that works — until too many play it.
Herd immunity requires about 90 percent vaccine coverage. Some children with highly vulnerable immune systems — say, a child being treated for leukemia — can't be vaccinated for medical reasons. When the number of non-medical exemptions from vaccination gets large enough, the child with leukemia becomes the most vulnerable to the spread of disease.