People who may be at risk of measles are being urged to have the MMR vaccine following an outbreak in south west Wales.
There have been a reported 189 cases of the infection in Swansea and Neath-Port Talbot since November, more than the whole of Wales in the past three years.
Doctors say anybody who is not sure whether they have had measles or been vaccinated should have the MMR jab.
They said there is no upper age limit for the vaccine.
The outbreak so far only affects the Abertawe Bro Morgannwg University Health Board area, where 32 secondary schools, primary schools and nurseries are affected.
Public Health Wales estimates there are more than 8,500 schoolchildren at risk of measles in the area.
Dr Matthew Snape, a consultant in vaccinology and general paediatrics from Oxford University, said anybody who had not had the MMR jab was at "serious risk".
He said many children missed out on the vaccine following the now-discredited 1998 report linking the MMR jab and autism and the vaccination rate dropped from 90% to 80%. It is now back to around 92%.
Children should receive the first dose of the vaccine at 12 to 13 months of age and the second at three years and four months of age.
But Dr Snape he said it was not too late to have the vaccine: "There's no upper age limit so parents can still take their child to the GP. That's what they should be doing to be honest, especially if it is now circulating again.
"Measles is still a very dangerous infection. It's not something that's a right of passage of childhood."
Swansea GP Dr Ian Millington said the vaccination was the only way to prevent measles.
"It's highly contagious and we only have one tool in the box and that's immunisation," he said.
"So I think any child who's not had two doses of MMR should be seeking that as quickly as possible."
Adults who did not know whether they had had the illness or been vaccinated could also have the MMR: "If in doubt, have your immunisations," he said. "There's no evidence to say that you can overdose with the MMR vaccine."
He added that there was nothing to fear from any side effects from the MMR: they include an "occasional sore arm" or "very, very occasionally a very slight rash".
'Severe brain damage'
"They are of no significance compared with the disease you're vaccinating against," Dr Millington added.
Dr Marion Lyons, director of health protection for Public Health Wales (PHW), said they were "concerned" about the outbreak.
"We cannot emphasise enough that measles is an illness that can kill, or leave patients with permanent complications including severe brain damage, and the only protection is two doses of the MMR vaccination," she added.
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 to the rest of the body over several days.
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