Health officials say a measles epidemic is "spreading at an alarming rate across areas of Wales".
The warning comes as latest figures show the number of cases in the Swansea area have more than doubled in the last three weeks.
Public Health Wales say there are 432 cases in total, of which 116 have been reported in the last week alone and 51 people have been hospitalised.
Parents are being urged to get their children vaccinated with MMR jabs.
Although cases have been reported across Wales, the majority are in Abertawe Bro Morgannwg, Powys and Hywel Dda health board areas.
But the outbreak of the highly infectious and potentially fatal virus shows no signs of fading, PHW says.
The authority says the outbreak has not yet peaked and if it continues to rise at the same rate, there could be about 1,000 cases by the end of April.
Dr Marion Lyons, director of health protection for PHW, said: "Measles is now spreading at an alarming rate 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.
"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."
Dr Lyons added the risk to unvaccinated children is increasing as the disease spreads.
Typical symptoms of measles include fever, cough, conjunctivitis and a rash.
Complications are quite common even in healthy people, and around 20% of reported measles cases experience one or more complication.
These can include ear infections, vomiting and diarrhoea, pneumonia, meningitis and serious eye disorders.
Dr Lyons added that children who have not been fully immunised face a life-long risk of catching measles but said one dose of MMR would protect 90% of children very quickly and two doses protects 99%.
She added: "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, think about it today, and 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."
Dr Lyons last week warned that the current outbreak is likely to continue into the summer.
About 200 people last week had their children vaccinated but PHW said more needed to be.