Measles vaccinations are being given to pupils at schools in parts of mid and south Wales.
It's after an outbreak of the virus in Swansea, which has now affected more than 800 people.
Measles is a viral illness that is easily spread by getting close to someone else who is infected.
About 10 years ago, some parents chose not to give their children the measles jab because there were worries it might have harmful side effects.
Doctors are now warning that up to 1 million kids in England could be at risk of measles.
Why have so many kids not had the vaccine?
In the late 1990s there were "scare stories" in the media about the MMR jab.
In 1998, Doctor Andrew Wakefield published claims that the vaccine was linked to a condition called autism in kids.
His research has since been shown to have major flaws and there is no proof that MMR is dangerous.
Mr Wakefield has been told he can't practice as a doctor any more.
"Disgraced doctor Andrew Wakefield's discredited and inaccurate research caused great harm to the MMR vaccination programme," Health Secretary, Jeremy Hunt said on Tuesday.
It "led to thousands of parents choosing not to vaccinate their children against measles, mumps and rubella."
What is measles?
Measles can make you feel really sick. It starts off feeling like a really bad cold.
But as it goes on it causes a red-brown rash to appear on the skin and can make your tummy hurt and give you diarrhoea.
Symptoms usually last about two weeks, but in rare cases it can lead to further problems for kids who catch the disease.