This outbreak of measles is the largest to occur in 15 years affecting 214 children so far. Typically there are 60 to 70 cases per year.
The Centers for Disease Control and Prevention says it believes this is likely driven by travelers returning from abroad and by too many unvaccinated children.
There is a growing trend among some US parents to skip the MMR vaccine for their children.
Experts say many parents have a misguided fear that the measles mumps and rubella vaccine is not safe.
Health officials say so far small outbreaks are being contained. But, they add, parents who decide against vaccinating their child may affect the health of other children.