The UK has lost its measles-free status three years after the virus was eliminated in the country.
Cases of the potentially deadly infection have been going up, with too few people being vaccinated.
Measles can be prevented through two doses of the MMR vaccine - something that is offered free by the NHS to all young children in the UK.
Why does it matter?
Measles is one of the world's most contagious diseases. It is a viral illness spread by coughing and sneezing. Although most people who catch it will recover, it can lead to life-threatening complications.
Before a vaccine was introduced, the UK used to see hundreds of thousands of cases and 100 or more deaths each year.
The measles vaccine, which is included in the MMR jab, is a safe and effective way to stop the disease.
Ideally everyone should be immunised, but the target set by health experts is at least 95% of the population vaccinated.
The UK achieved this in 2016, when the World Health Organization declared the nation measles-free.
That did not mean that measles had been wiped out entirely, just that none of the cases originated in the UK.
But that has now changed. The UK has lost its eliminated status and measles cases are on the rise.
What's going on?
Two doses of the vaccine leaves 99% of people protected - the majority of new measles cases have been in people who are not vaccinated. These cases are most common among older teenagers and people in their early 20s, who missed out on MMR vaccination when they were younger.
A single MMR shot was introduced in 1988, with a second dose added to the immunisation schedule in 1996. Before that, people used to have an individual measles jab. MMR was introduced to improve uptake, coverage and protection.
Today, some parents refuse vaccination for their children, despite the overwhelming evidence in favour of the vaccine and the extensive public health campaign.
Measles circulates in many countries around the world, and there are currently several large outbreaks across Europe in countries where rates of MMR vaccination have been low.
Experts say that until measles elimination is achieved globally we will continue to see cases of measles coming to the UK.
In 2018, there was a marked increase in reported measles cases, with 991 confirmed cases in England and Wales, compared with 284 cases in 2017.
And there have been more than 230 cases across the UK during the first quarter of 2019.
Most are linked to travel in Europe. There have been particularly serious outbreaks in Serbia, Ukraine, Georgia, Greece, Romania, Italy and France.
This can lead to outbreaks in communities and areas where MMR vaccination coverage is below the 95% needed to achieve widespread protection.
Cases have also been linked to young people (who missed out on their MMR vaccine in childhood) attending music festivals and other large public events.
The NHS offers catch-up immunisation programmes for people who have not had their two doses of MMR.
Why are some children not being vaccinated?
Experts say part of the explanation may be complacency - when measles cases became less common people began to perceive the threat of the disease to be less.
The publication of a controversial and since discredited piece of research in the late 90s by a doctor called Andrew Wakefield wrongly linking MMR with autism is another factor.
It led to a drop in parents taking their children for the jab, and rates of vaccination took many years to recover. It continues to have an impact today.
There is much misleading information on social media and it can be quickly spread.
What can be done?
Prime Minister Boris Johnson has said he wants urgent action to boost vaccination rates.
He is asking social media companies to "play their part" in promoting accurate information on vaccination, and help address any concerns or questions that parents or the public might have.
Some internet companies have attempted to address this problem.
This year Twitter introduced a function which directs users towards "a credible public health resource" when they search for certain keywords including measles, MMR and vaccinations. In the UK, users are prompted to visit the NHS website.
Facebook said in April that it planned to reduce the ranking of pages that spread misinformation about vaccination, making them harder to find.
A check of MMR status for 10 and 11-year olds has recently been added to the GP contract in England and the government's recent Green Paper on prevention looks at how to increase rates of vaccination.
Earlier this year, Health Secretary Matt Hancock - who controls the NHS in England - said that he was willing to look at "all options" to boost vaccination levels. These would include compulsory immunisation - like some other countries, including Italy, where access to school places requires vaccination.
What's happening around the UK and elsewhere?
As measles is highly infectious, even small declines in vaccination rates can have an impact, and anyone who has not received both doses of MMR is at risk.
Just 87% of children in England are receiving their second dose of MMR. In Wales and Scotland it is around 92%, in Northern Ireland it is 91%. The target is 95%.
In London, estimates suggest about one in four children starting primary school do not have the full protection that the MMR vaccine offers.
For several years global coverage with the first dose of measles vaccine has stalled at 85%. Second dose coverage, while increasing, is 67%.
An estimated 20 million measles cases and 4,500 deaths have been prevented in the UK since a measles vaccine was introduced in 1968.