A campaign has been launched to find kids in England who have missed out on important injections that stop you getting measles.
There are fears that there could be a new outbreak of the nasty spotty disease affecting loads of kids that aren't protected against it.
Three million mums and dads will be getting letters urging them to get their kids vaccinated.
Doctors are being given extra vaccinations for the campaign.
Kids usually get the anti-measles jab at about 18 months and the injection also protects against mumps and rubella.
Lots of kids didn't get the anti-measles jab because of fears that it was linked with autism, but that has been proved not to be true.
Measles had nearly been wiped out but since some kids stopped having the jabs, there's been a rise in the number of cases.