Cases in the ongoing measles outbreak have risen to 258 across Texas, New Mexico and Oklahoma, and state health departments are urging more people to get the measles-mumps-rubella vaccine.

As a measles outbreak has grown in Texas and New Mexico, doctors and public health experts have stressed that the measles, mumps and rubella vaccine is the most effective protection against the highly contagious respiratory virus.

It was considered a public health triumph 25 years ago, when the U.S. declared measles eradicated. But in recent years, the virus has made a comeback- in part due to an increase in international travel.