At least 40 people have died and many thousands infected with the measles virus after an extended outbreak hit Europe.
More than 41,000 children and adults have been struck down by the illness in the first six months of 2018, according to the World Health Organisation.
The WHO said the total number of cases far exceeds the annual totals reported for every other year this decade.
“Following the decade’s lowest number of cases in 2016, we are seeing a dramatic increase in infections and extended outbreaks,” WHO Regional Director for Europe Dr Zsuzsanna Jakab said.
“We call on all countries to immediately implement broad, context-appropriate measures to stop further spread of this disease.”
Seven countries, including France, Georgia, Greece, Italy and Ukraine, have seen more than 1000 infections in children and adults this year alone.
The Ukraine was the hardest hit, with more than 23,000 people affected.
The measles virus is exceptionally contagious and spreads easily among susceptible individuals.
WHO is working closely with member states facing outbreaks to implement response measures.