A vaccine is a substance that preemptively provides immunity against a given disease. Countries around the world launch national vaccination programs to make people healthier, and prevent the spread of certain diseases.
Yet some people refuse to take vaccines because they think they may have side effects that bring about more damage than protection.
"Anti-vaxxers" is the name given to such people.
The co-ruling anti-establishment Five Star Movement in Italy has questioned the safety of some vaccines and loudly denounced efforts to make vaccinations mandatory.
However, such a trend has already led to grave consequences among Europeans, as WHO had stated.
"Progress has been uneven between and within countries, leaving increasing clusters of susceptible individuals unprotected, and resulting in a record number of people affected by the virus in 2018," the statement said.
Measles is a viral disease that can damage the brains of children and even kill them.
"(Immunization) gaps at local level still offer an open door to the virus," WHO's European Director Zsuzsanna Jakab said in a statement.
Countries outside Europe are also suffering from the outbreak.