The UK eliminated measles in 2017 but now cases are increasing once more, putting at risk those who haven't been vaccinated - including those who can't be jabbed for medical reasons.
With experts saying that measles is more contagious than Covid, we take a look at why measles is more serious than some people might think, and the symptoms you need to look out for.
Since March 2020 and the start of the Covid pandemic, there's been a steep drop in the number of children being vaccinated against MMR and other childhood vaccinations at the correct time.
Coverage of the first dose in children aged two has dropped below 90% and coverage for the two doses in five-year-olds is 85.5%.
Although this may sound like a lot, it's below the 95% needed to achieve and sustain measles elimination, according to the World Health Organization (WHO).
This means that more than one in 10 children aged under five aren't fully protected from measles and are at risk of catching (and spreading) it, which is why the government is urging those who've missed out on their vaccines during the pandemic to have them now.
Measles is an extremely infectious viral disease spread through water droplets coughed or sneezed by those with the infection.
It can affect anyone who hasn't been vaccinated, whatever their age.
You are infectious for four days from when the first symptoms appear, so stay at home if you think you have it to reduce the risk of spreading it to vulnerable people (including babies and those with weakened immune systems).
Make sure to drink plenty of fluids and take paracetamol or ibuprofen to lower high temperatures.
Measles is even more contagious than a virus that has changed all of our lives in the past two years - coronavirus.
Molecular virologist Dr Phil Gould of Coventry University says: 'Measles is definitely more contagious than Covid. Whereas the coronavirus has an R number of around two or three, measles has an R number of 15.'
This means that, on average, every person with measles will spread it to 15 others; a person with Covid will spread it to around three people.
You only need to be in contact with someone with measles for 15 minutes to get infected.
Parents may view measles as 'just another childhood illness' similar to chicken pox.
But Dr Gould say: 'While chicken pox may cause discomfort, measles can have serious consequences.'
These can include:
The measles virus also causes immune system damage that can take up to three years to recover from.
In some rare cases it can cause devastation years later with sclerosing panencephalitis (SSPE), which leads to the progressive destruction of the central nervous system, dementia, loss of motor control, epilepsy and eventually death.
Before a measles vaccine became available in 1968, it's estimated that measles was the direct or indirect cause of over half of all childhood deaths from infectious diseases.
Since then the vaccine has prevented 20 million measles cases and 4,500 deaths.
'Herd immunity' means that if enough people are vaccinated, it's more difficult for the disease to spread to those who can't have vaccines.
However, herd immunity against measles requires about 95% of a population to be vaccinated (rather than the current 85.5%) in order to protect the remaining 5%, which includes: