Measles outbreaks seem to be accelerating, and in the last five months have caused more U.S. illnesses than in any entire year since 1996.
Health officials say 307 cases have been reported since New Year's Day. About half have been in the past month - most from a huge outbreak in unvaccinated Amish communities in Ohio.
Nearly all the cases have been linked to travelers who caught the virus abroad and spread it in the United States among unvaccinated people. Many of the travelers had been to the Philippines, where a recent measles epidemic has caused more than 30,000 illnesses.
Most of the unvaccinated skipped shots for personal or philosophical reasons.
The Centers for Disease Control and Prevention released the new numbers Thursday.