People who have not had the MMR jab are being urged to come forward
An outbreak of measles among children on a housing estate in Merthyr Tydfil, south Wales is under investigation.
Twelve cases have been reported in the last two weeks on the Gellideg estate and more are suspected.
The National Public Health Service (NPHS) for Wales said none of those who are ill have had the MMR vaccine.
Wales is currently undergoing the most serious outbreak of measles since the jab was introduced with 371 cases reported in 20 of its 22 counties.
The NPHS is urging parents whose children are not vaccinated against the potentially fatal illness to take action immediately.
They have written to GPs across Merthyr Tydfil and Rhondda Cynon Taf to ask them to look out for patients with symptoms and to help encourage people to get vaccinated.
Dr Gwen Lowe, Consultant in Communicable Disease Control for the NPHS, said: "This is the first true cluster of measles that we have seen in Merthyr for many years.
"It is affecting children of both primary and secondary school age and we are concerned that the outbreak will quickly spread beyond the estate.
"Given the alarming increase in measles across Wales this year, we have continually urged parents to ensure their children have received the full two doses of the MMR vaccine, but parents in the Merthyr Tydfil area now need to take particular care to ensure their children are protected."
Children should have their first MMR vaccination at 13 months and the second at around three years and four months.
Anyone in Wales who should have received two doses of MMR but has missed out remains at risk of catching measles.
Dr Lowe added: "For as long as there are children who do not receive their MMR vaccinations, there is the potential for people to become ill with measles.
"It is a serious disease and sadly children do die from this infection. Complications are common and rarely can include brain damage."
"The people most at risk of catching measles are children of school age or children between the ages of one and four who are not up-to-date with their vaccinations.
"Although we are seeing cases of measles in adults, it is rare for anyone born before 1970 to catch measles."
Many people who catch measles will have a fever, cough, red eyes, and blocked nose and feel generally unwell. The blotchy rash appears a few days later beginning on the face and spreading downwards over several days.
Typically, people will be infectious from the day before their first symptoms until four to five days after the rash appears.
Dr Lowe said: "If your child is unwell and you suspect it is measles, you should contact your GP. Your child should not attend school or nursery for five days after the rash starts.