A dozen children have contracted measles in Cardiff - the first outbreak of the disease in the capital in 10 years.
Public health doctors urge parents to give their children the MMR jab
The ages of the children - one of whom is in hospital - range from seven months to 10 years and their infections are all linked, say health experts.
All the cases involve youngsters in the east of the city who have not been vaccinated, leading public health doctors to urge parents to give their children the MMR - measles, mumps and rubella - jab.
Dr Marion Lyons, Consultant in Communicable Disease Control at Bro Taf Health Authority, said: "Measles is a serious illness and it can spread rapidly among children.
"Some children who catch measles may have serious complications and can be sick for a long time or even die.
"There is a very effective vaccination against measles, which is the MMR.
"A full course of MMR is two vaccinations and will give the child lifelong protection against measles and also against mumps and rubella (German measles).
"We strongly recommend that if any child is not protected by two doses of MMR that parents have them vaccinated now."
There is a very effective vaccination against measles, which is the MMR
Vaccination rates in Bro Taf Health Authority, like those across Wales, have been falling since the controversial claim in 1998 that the combined MMR vaccine was linked to the development of autism in some children.
However, no research has yet to provide conclusive proof that such a link exists and a number of studies have suggested there is no link between the jab and autism.
The latest available figures show the MMR take-up rate in the health authority area has fallen to 85.7% of children, down from 87.4% in January last year.
The average take-up rate for MMR in Wales is 83.8%.
The Neath Port Talbot and Swansea region has the lowest take-up rate at 72%.
Take-up of MMR jab has fallen after fears about its safety
Doctors there have warned of a measles outbreak and the area's health authority has launched an awareness campaign to encourage more parents to immunise their children.
The World Health Organisation recommends 95% of children in a population need to be vaccinated to prevent the disease from spreading.
Measles remains a killer in the developing world, taking the lives of around one million children worldwide.
The number of cases in England and Wales has tripled in recent years and is at its highest since the MMR vaccine was introduced in 1988.
Experts say it once again raises concerns that the fall-off in numbers of children having the combined vaccine will lead to increasing outbreaks of the disease.
The symptoms of measles are feeling unwell, fever, sore eyes, cough and red blotchy rash, which spreads from the face all over the body.
If a child develops these symptoms, a doctor should be contacted.