A measles outbreak has been declared at a school in Newport.
Public Health Wales said four people with links to Lliswerry High School had been diagnosed with the illness.
Vaccinations are being carried out at the school and parents have been sent letters to alert them to the risk.
Parents have been urged to ensure their children have received both doses of the MMR vaccine. The first dose is usually given at 12 months and the second at 40 months.
Symptoms include a fever, cough, runny nose, conjunctivitis and a distinctive red rash.
Heather Lewis, consultant in health protection for Public Health Wales, said: "While four cases of measles may not sound a lot, we know that there are other children attending the school who are unvaccinated and could easily catch and spread measles."
Adults who have never had measles or the MMR vaccine and who work in close contact with children have also been urged to speak to their GP about vaccination.