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.
First to Swansea in South Wales, and the measles outbreak which has now affected more than 800 people. Health officials are going into local schools this week to offer vaccinations to pupils. Measles is easy to catch from someone who's got it, and doctors think up to a million kids in England could be at risk, because they didn't get the jab when they were little... (RUN VT FF)