There has been increasing debate among parents over whether or not to vaccinate their kids. As Shana alluded above, as parents elect to forego vaccinations, the risk of disease contraction rises for the whole population. This is an example of a tragedy of the commons.

From the ABCnews article linked to above,

"Researchers have found that vaccination rates of at least 93 percent are needed to ensure so-called herd immunity against pertussis, which prevents the disease from spreading quickly to unvaccinated individuals."

For more information, see the Center for Disease Control's webpage on vaccines and immunizations.