More than half of U.S. travelers who should get the measles, mumps, and rubella vaccine before they go abroad are not getting the shot, putting many others at risk when they return home, a new study finds.

The research, published today in the Annals of Internal Medicine, is important, experts say, because most measles outbreaks in the United States can be traced back to returning travelers who become infected overseas.

“There continues to be measles importations into the U.S. which then can provoke measles outbreaks in different communities,” lead author Emily Hyle, M.D., an infectious disease physician at Massachusetts General Hospital, told CBS News.

Measles can spread like wildfire. A single case of imported measles can wind up sickening a dozen or more unvaccinated people.

“Measles is incredibly contagious. Ninety percent of non-immune individuals will become ill with measles if they’re exposed,” Hyle said. “And that exposure can be as minimal as walking into a room up until two hours after somebody infected with measles has been there.”

Since 1989, clinical guidelines from the CDC Advisory Committee on Immunizations have called for all U.S. international travelers to be vaccinated with two doses of MMR vaccine before their trip unless a blood test shows they have measles immunity, they had a documented case of the measles, or they were born before 1957 (when the disease, and resulting immunity, were widespread in the U.S.).