Public health officials are investigating 15 cases of measles in Carmarthenshire and Cardiff.
Thirteen students and staff at Newcastle Emlyn Comprehensive School have been diagnosed with the illness.
Two family members of one of the pupils who live in Cardiff have also caught measles, the National Public Health Service for Wales (NPHSW) said.
The cases are believed to be linked and pupils who have not had the MMR vaccination are now being offered it.
Letters have been sent to others believed to have come into contact with those diagnosed.
Parents are being urged to be aware of the symptoms of measles and to arrange for their children to receive the two dose course of the MMR vaccination if they have not been immunised yet.
At risk
Dr Roland Salmon, director of the communicable disease surveillance centre at the NPHS, said: "Any upsurge in measles case is of concern to us as this is a potentially serious disease that can be fatal or can have permanent consequences such as brain damage.
"Children normally receive the first dose of the vaccine at the age of one, and the second before they start primary school.
Dr Salmon added: "Parents who think their children have missed one or both doses of the vaccine should speak to their GP or health visitor immediately as, without vaccination, children are at risk."
Measles is highly contagious and is generally spread from person to person by coughs and sneezes.
Early symptoms include a high temperature, cough and conjunctivitis and a red, blotchy rash that appears about three days later, usually starting on the face and spreading down over the rest of the body.
The NPHS said any child who develops symptoms of measles should not attend school for five days after the rash begins.
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