Measles cases more than tripled in Europe last year, with outbreaks and epidemics fueled by low vaccination rates, the World Health Organization says.
More than 21,000 people caught the virus in 2017, and 35 people died of it, WHO said. The numbers stunned officials after a record low of 5,200 cases in 2016.
Advisers met Tuesday to talk about how to increase vaccination rates in Europe.
"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," Dr. Zsuzsanna Jakab, WHO Regional Director for Europe, said in a statement.
"Over 20,000 cases of measles, and 35 lives lost in 2017 alone, are a tragedy we simply cannot accept."
RELATED: A look at the measles
UNDATED: In this handout from the Centers for Disease Control and Prevention (CDC), a thin-section transmission electron micrograph (TEM) reveales the ultrastructural appearance of a single virus particle, or 'virion', of measles virus. in this undated image. Measles outbreaks have been reported throughout the U.S., with the latest reported February 5, 2015 at a daycare in suburban Chicago where as many as five children under the age of one have been infected. (Photo by CDC via Getty Images)
MILL VALLEY, CA - JANUARY 26: In this photo illustration, vials of measles, mumps and rubella vaccine are displayed on a counter at a Walgreens Pharmacy on January 26, 2015 in Mill Valley, California. An outbreak of measles in California has grown to 68 cases with 48 of the cases being linked to people who had visited Disneyland. Nine additional cases have been reported in five states and Mexico. (Photo by Illustration Justin Sullivan/Getty Images)
MIAMI, FL - JANUARY 28: Miami Children's Hospital pediatrician Dr. Amanda Porro, M.D prepares to administer a measles vaccination to a child at the Miami Children's Hospital on January 28, 2015 in Miami, Florida. A recent outbreak of measles has some doctors encouraging vaccination as the best way to prevent measles and its spread. (Photo by Joe Raedle/Getty Images)
1972: In this handout from the Centers for Disease Control and Prevention (CDC), a histopathology of measles pneumonia is seen in this microscope image from 1972. Measles outbreaks have been reported throughout the U.S., with the latest reported February 5, 2015 at a daycare in suburban Chicago where as many as five children under the age of one have been infected. (Photo by CDC via Getty Images)
MIAMI, FL - JANUARY 28: In this photo illustration, a bottle containing a measles vaccine is seen at the Miami Children's Hospital on January 28, 2015 in Miami, Florida. A recent outbreak of measles has some doctors encouraging vaccination as the best way to prevent measles and its spread. (Photo illustration by Joe Raedle/Getty Images)
A girl receives anti-measles vaccination drops at a health centre in BASECO compound in Tondo, Manila September 3, 2014. Philippine President Benigno Aquino said on September 1, 2014 between 11 to 13 million people in the country are at risk from measles, polio and rubella (German measles), and asked the public to cooperate in eradicating the preventable diseases, during Monday's launch of a mass national vaccination campaign against measles and polio, reported local media. REUTERS/Erik De Castro (PHILIPPINES - Tags: HEALTH SOCIETY)
A measles vaccine is seen at Venice Family Clinic in Los Angeles, California February 5, 2015. Lawmakers in several U.S. states are backing proposals to make it harder for parents to opt out of school vaccinations based on personal beliefs, as health officials fight a growing measles outbreak that has sickened more than 100 people in more than a dozen states. REUTERS/Lucy Nicholson (UNITED STATES - Tags: HEALTH POLITICS)
A girl looks on as a health officer injects her with measles vaccine at a school in Yemen's central province of Marib January 12, 2016. REUTERS/Ali Owidha
ACEH, INDONESIA - NOVEMBER 14: Students receive immunizations against TD (Tetanus Toxoid) and DT (Diphtheria Tetanus) during the implementation of the School Children Immunization Month in Lhokseumawe, on November 14, 2016 in Aceh, Indonesia.
TD immunization shots (Tetanus Toxoid) and DT (diphtheria tetanus) to prevent measles, diphtheria and tetanus, which aims to improve the health of students in Indonesia.
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AMAZON, MANAUS, AMAZONAS STATE, BRAZIL - 2016/03/16: Measles, a highly contagious infection caused by the measles virus is an airborne disease which spreads easily through the coughs and sneezes of those infected - red rash which usually starts on the face and then spreads to the rest of the body is the typical symptom. Fundacao de Medicina Tropical do Amazonas ( Amazon Tropical Medicine Foundation ), Manaus city, Brazil. (Photo by Ricardo Funari/Brazil Photos/LightRocket via Getty Images)
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Europe has been struggling to control measles for years. One of the worst recent years was 2013, when there were more than 10,000 cases across Europe. But 2017 had more than doubled that number.
The worst recent year in the United States was 2014, when there were 667 cases of measles. Last year, 86 people were diagnosed with measles in the U.S. and so far this year 9 cases have been reported to the Centers for Disease Control and Prevention.
"The majority of people who got measles were unvaccinated," the CDC says. "Travelers with measles continue to bring the disease into the U.S. Measles can spread when it reaches a community in the U.S. where groups of people are unvaccinated."
That's what is happening in Europe, as well. The CDC has issued travel advisories about Europe, reminding Americans to make sure they are up to date on the measles, mumps and rubella (MMR) vaccine.
Experts say that to control measles, 95 percent of the population must be vaccinated or immune because of a past infection. Measles is one of the most infectious viruses — it infects 90 percent of people who are exposed to it if they aren't immune, and it can hang in the air and infect people who enter a room as long as two hours after an infected person has left.
But social media has helped spread incorrect information about vaccine effectiveness and safety. Vaccine critics have helped lower rates in Italy, the country's health authorities say. Vaccination rates fell from 90 percent in 2010 to 85 percent in 2015.
And countries with fewer resources to provide vaccines have trouble getting them even to people who want them.
WHO said Romania had the most cases last year with 5,562. Italy had more than 5,000 cases and Ukraine had 4,767 cases.
It says more than 20 million people were infected with measles in 2016.
Despite the increases in Europe, WHO says vaccination against measles is a success story.
"Global measles deaths have decreased by 84 percent worldwide in recent years — from 550,100 deaths in 2000 to 89,780 in 2016," WHO says.