(CNN) - Health officials in Washington state have declared a state of emergency over the measles.
The Centers for Disease Control and Prevention is outlining the scope of the outbreak, and detailing what can be done to stop it in its tracks.
Dr. Nancy Messonnier, the director at the CDC National Center for Immunization and Respiratory Diseases, said it’s “those that are unvaccinated that put us all at risk."
Measles cases are continuing to spike in Washington, with the highest number of infections there in over two decades.
More cases are starting to pop up in states across the U.S.
It’s a concerning comeback for a disease that's been declared eradicated in the U.S.
"Americans who travel abroad might be surprised to find out that a number of the countries where measles is being transmitted are countries that would be surprising to them,” Messonnier said.
The CDC’s travel notice list spans from European countries like England, France, Greece and Italy to Asian nations like the Philippines and Indonesia, to South American destinations like Brazil and Colombia.
Health officials traced the current outbreak in Washington to an unvaccinated international traveler. They also said they’re seeing about five new cases each day.
In New York, they’ve had more than 200 cases since October. Most of those have come from observant Jewish communities linked to travel to Israel, another country on the notice list.
Health officials said groups with low vaccination rates are the most at risk.
"Americans who are unvaccinated put themselves, their families and their communities at risk for measles,” Messonnier said.
There are 18 states that allow parents to opt out of vaccinations for personal beliefs.
The CDC’s position, though, is that there’s no acceptable reason not to get them.
“There aren’t any other alternatives to vaccination,” Messonnier said. ”The best way to stop this outbreak in its tracks is to make sure that everybody gets vaccinated.”