Parents who don't get their children vaccinated against measles are causing the disease to 'spread like bush fire' in one area of the country.
Health experts in Wales have warned the epidemic could leave unprotected children brain-damaged or dead.
Dr Marion Lyons, Public Health Wales director of health protection, warned: "Measles is now spreading like bush fire across areas of Wales.
"Worryingly, there are still tens of thousands of susceptible children across Wales, yet our weekly monitoring of vaccination rates shows only a slight increase in numbers receiving MMR jabs."
She added: "If the numbers of parents bringing their children for MMR jabs does not dramatically increase, measles will continue to spread and quickly reach levels last seen in the outbreak in Dublin in 1999/2000. In that outbreak over 1,200 children were infected and three died.
"The risk to unvaccinated children in Wales is increasing as the disease spreads. You only need one or two people who haven't had the vaccination in a community to put at risk babies, toddlers and anyone else who is vulnerable, such as children with leukaemia who cannot have the vaccination and pregnant women who haven't been vaccinated.
"A simple and safe jab from your GP will protect your child's health, could save their life, and will help protect other children too.
"It is the only precaution you can take at this time, and I would urge parents to contact their GP today to get their children's MMR vaccinations up to date.
"If you haven't thought about it, make an appointment with your GP for your child to get the MMR jab.
"If you're worried about the jab, I would reassure you that the jab is safe and offers the only protection against measles. Get your child vaccinated."
Parent Joanne Jones told BBC Wales Today: "I know someone who didn't immunise. Her son is in the same class as our son and one of the little boys has been hospitalised recently and is quite seriously ill with measles.
"She has taken all her children out of school because they haven't been immunised and is paying privately to have them immunised."
The latest warning is fuelled by concerns that increasingly urgent public pleas for parents to act do not appear to be getting through. The outbreak, centred on Swansea and the surrounding area, has seen the numbers infected rise from just over 200 on March 4 to 432 now.
By the end of last week, the disease had spread to children in 111 secondary and primary schools, nurseries and play groups.
Experts with Public Health Wales (PHW) expressed rising concern last week when the number of new cases rose. Since then, a further 116 cases have been reported and the rate at which the disease is spreading shows no sign of slowing.
The majority of cases so far are in the Swansea area but an increasing number are being reported across the mid and west Wales regions.
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Measles: Symptoms to spot and how to protect your child