Measles cases soared in Europe last year, and at least 35 children died of the highly infectious disease, according to the World Health Organization.

The virus found its way into pockets of unvaccinated children all over the continent, from Romania to Britain. The number of recorded cases quadrupled, to 21,315 in 2017 from 5,273 in 2016, a record low.

The biggest outbreak last year was in Romania, where there were 5,562 cases and which accounted for most of the deaths. The country’s large rural Roma population — also known as Gypsies — often do not vaccinate their children and may not take them to hospitals promptly when they fall ill. The country also has an underfunded public health system.

The second biggest outbreak was in Italy, with 5,006 cases and three deaths; 88 percent of those cases were in people never vaccinated, and another 7 percent in people who had not had all the recommended doses, the European Center for Prevention and Disease Control said.