Due to measles outbreaks in Europe, experts strongly recommend vacationers going abroad are vaccinated.

If you travel abroad this summer, it’s likely you’ll bring back souvenirs and some vacation photos.

However, if you are unvaccinated and aren’t careful when you’re overseas, you might also bring back a case of the measles.

That’s particularly true this summer with measles outbreaks happening in Europe and elsewhere.

It also comes in light of a recent study that showed 16 percent of international travelers from the United States in recent years needed a measles vaccine, but 50 percent of them declined to get one.

The situation has officials at the U.S. Centers for Disease Control and Prevention (CDC), as well as other agencies, worried.

“This is not only to protect themselves, it’s also to prevent the importation of the disease,” Dr. Gary Brunette, the branch chief of the travelers’ health division at the CDC, told Healthline.

Read more: Get the facts on measles »

The recent study was published this month in the Annals of Internal Medicine.

In it, researchers sifted through data collected at 24 GlobalTravEpiNet clinics between 2009 and 2014.

The researchers said 40,810 people were included in their analysis. Of them, 16 percent were determined to be eligible to receive the measles, mumps, and rubella (MMR) vaccine.

However, slightly more than 50 percent of those travelers decided not to get inoculated.

Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center, said these unvaccinated travelers are taking a big risk.

“It’s the single most contagious virus we know,” he told Healthline. “You also get quite sick for a few weeks. It’s a terrible disease.”

“It’s a real risk for travelers,” added Brunette.

Schaffner and Brunette both said the additional concern is that these travelers return to the United States before symptoms appear and then spread the highly contagious ailment.

They said many of these unvaccinated travelers live in “pockets” where large numbers of a community have decided to forego immunization.

In such areas, measles can spread quickly to unvaccinated people. It can also endanger people who can’t be inoculated, such as children undergoing cancer treatment.

The outbreaks can also lower the overall “herd immunity” a community can develop with a high vaccination rate.

“The bottom line is if they don’t bring home measles, then we won’t have it here,” said Schaffner.

Read more: Rise in measles, whooping cough due to unvaccinated people »

Measles cases in the United States are relatively low due to the country’s high vaccination rate.

In 2016, there were only 70 reported cases of measles in 16 states. That compares to the more than 3 million people in the United States who used to contract measles every year before the measles vaccination program was instituted.

When there is an outbreak, it usually starts with someone who has brought back the disease after traveling aboard.

That was the case in January 2015 when close to 60 people in California came down with the ailment. More than 40 of those measles cases were linked to visits to Disney theme parks in Southern California, where a person who traveled overseas was believed to have brought the virus back.

Schaffner and Brunette both pointed out that few countries in the rest of the world have the high vaccination rate and low measles caseload that the United States does.

Even in western Europe, countries are laxer about immunization. France, for example, reportedly has the most skeptical population toward vaccinations in all of Europe.

In fact, there has been an outbreak of measles in Europe this year. The epidemic has become serious enough that last week German officials announced that parents who don’t seek medical advice on vaccinating their children could face fines.

The situation in Europe coincides with the usual higher levels of cases on other continents.

Schaffner and Brunette said people planning vacations should visit either their doctor or a travel clinic several weeks before their departure to make sure they have all the vaccinations they need.

Advance planning is needed because some immunizations might require an additional shot, and you need to be sure you have time to get that in.

Besides an MMR inoculation, there may be a need for malaria, typhoid, or other vaccinations.

“I think a lot of people aren’t aware of these things,” said Brunette.

Read more: Some adults do need to be revaccinated against the measles »