Measles is highly contagious, but easily preventable with a vaccine.
However, the numbers of measles cases jumped sharply in Europe in 2017, according to new data released by the World Health Organization.
In 2017, the disease affected 21,315 people, compared with 5,273 in 2016. Last year, 35 people died in Europe because of measles.
"Every new person affected by measles in Europe reminds us that unvaccinated children and adults, regardless of where they live, remain at risk of catching the disease and spreading it to others who may not be able to get vaccinated," said Zsuzsanna Jakab, WHO's regional director for Europe. She describes the loss of lives as a "tragedy we simply cannot accept."
The WHO says that 15 countries in Europe saw outbreaks of 100 or more cases. Romania had the most, with 5,562, followed by Italy, with 5,006, and Ukraine, with 4,767.
"These countries have experienced a range of challenges in recent years, such as declines in overall routine immunization coverage, consistently low coverage among some marginalized groups, interruptions in vaccine supply or underperforming disease surveillance system," the WHO report stated.
According to the BBC, Romania has a shortage of the vaccine. "It is also thought that the country's large Roma population, who often live in severe poverty, are at particular risk of contracting and spreading the virus," the broadcaster added.
In response to the burgeoning outbreak in Italy last year, the country made vaccines mandatory for measles and a number of other diseases for schoolchildren.
"Kids up to 6 years old won't be accepted into nursery schools without them," reporter Christopher Livesay told NPR. "And parents sending their children to school after that age without vaccinating them first will now face fines of up to $8,380."
That was controversial and sparked protest. Some politicians in Italy, as in many other countries, have argued against the policy by referencing widely discredited research that suggests a link between vaccines and autism.
The WHO says other European countries that saw large outbreaks in 2017 are:
"Greece (967), Germany (927), Serbia (702), Tajikistan (649), France (520), the Russian Federation (408), Belgium (369), the United Kingdom (282), Bulgaria (167), Spain (152), Czechia (146) and Switzerland (105)."
As NPR's Michaeleen Doucleff has reported, measles cases have generally seen a dramatic drop worldwide since the 1980s, from more than 4 million cases annually to fewer than 500,000. However, she adds that health workers have struggled to push vaccination rates past 78 percent, though they say rates of 90 to 95 percent are needed to stop outbreaks.
Poor countries such as Guinea, Mongolia and Nigeria are seeing a high number of cases, she adds. As NPR's Jason Beaubien reports, Rohingya camps in Bangladesh have also seen recent outbreaks.
There have also been several recent outbreaks in the U.S., primarily impacting people who were never vaccinated for measles. The U.S. saw 118 cases of measles in 2017, according to the Centers for Disease Control and Prevention. A 2015 outbreak across multiple states is believed to have originated with a traveler who visited Disneyland.
An outbreak hit Minnesota in 2017, sickening at least 79 people. A large number of those affected are members of the Somali-American community in Minneapolis, many of whom are wary of vaccines.
Patients with measles have symptoms such as fever, cough and runny nose at the onset, according to the CDC. After about three days, a rash of "flat red spots" appears on the infected person's body.