The consequences of measles - a highly infectious disease - can be deadly.
Hundreds of thousands of deaths are reported worldwide each year. There were 72 deaths from measles in Europe in 2018, compared with 42 in 2017.
UNICEF says the disease is more contagious than Ebola, tuberculosis or influenza.
The virus can be contracted by someone up to two hours after an infected person has left a room. It spreads through air and infects the respiratory tract, potentially killing malnourished children or babies too young to be vaccinated.
Once infected, there is no specific treatment for measles, so UNICEF says vaccination is a life-saving tool for children.
RTÉ News spoke to Dr Robin Nandy, Principal Adviser and Chief of Immunisation at UNICEF Headquarters in New York.
Dr Nandy said: "Nowhere is the saying 'the prevention is better than the cure' more true than in the case of measles."
Dr Nandy explained why the measles outbreak is particularly high in areas where there has been conflict.
Dr Nandy said measles has affected countries such as Syria, where vaccination rates were traditionally very high, but fell after the conflict began.
He said the disease is one of the most highly infectious: "If you introduce it to a group of ten kids who are non-immune, nine of them are going to get the disease, so measles selectively seeks out these unvaccinated pockets and that's why you see a huge impact in these conflict affected countries with disrupted systems."
But Dr Nandy said that fear of vaccinations and the anti-vax movement has had a role to play in countries such as Ireland and the US, where cases are continuing to grow.
He said anti-vaccine sentiment has been around for a long time, but he warned that whilst social media helps to spread information, it is also effective in spreading misinformation.
International travel has added to the problem. This week, a newspaper in Costa Rica reported that a French boy on holiday with his mother had brought the disease there, following five years of having no reported cases
Dr Nandy said: "A virus as contagious as measles does not know any borders. Measles somewhere is a threat everywhere."
He said it was necessary to have an immunisation rate of 95% in any given country to prevent the disease from spreading there.