In parts of Ireland an outbreak of measles has so far left two babies dead and scores of children in hospital - and experts fear the same thing could happen in the UK.
There has been a sharp fall in the number of children receiving the measles, mumps, and rubella (MMR) vaccine.
In some places up to a quarter of children are not being immunised.
The fall is being blamed on a lack of public confidence in the MMR vaccine sparked by research suggesting that it could cause autism and bowel disorders.
Public health experts have dismissed the research, but widespread media coverage has left many parents unsure as to the benefits of getting their children vaccinated.
It was in December last year that Dr Peter Keenan, head of casualty at the Temple Street Children's Hospital, began to see cases of measles.
He said: "I have not seen measles here since I was here as a registrar in the late seventies."
By the end of April the hospital had seen 313 children and babies with measles.
More than 100 children have had to be admitted to the hospital, and eight needed intensive care.
Very sick
Clinical nurse manager Anna Dermody said: "These children needed really acute intensive care nursing care."
Professor Donald Gill, a senior paediatrician at the hospital, was surprised at just how sick the children were.
He said: "They have a very florid rash that covers their body.
"They are extremely miserable, they just lie down, they want to be left alone, they want to sleep. They won't eat, they won't sleep. They run high fevers.
"Many of them have had a really severe cough, and developed pneumonia. Indeed we have had to ventilate three children - which is a very big surprise."
One child was dead when they arrived at hospital, another died despite hospital treatment.
Dr Mary Cafferky, a consultant in microbiology, blamed a vaccination rate of about 70% in Dublin for the outbreak.
She said many cases had affected children aged under 15 months.
The MMR vaccine is usually given to children in Dublin at 15 months, and Dr Cafferky says measles would not have spread to the younger children if the level of MMR uptake had been sufficiently high.
She said: "It is appalling, it is preventable, it should not be happening.
"People have forgotten that measles is a severe multi-system illness - that is why the vaccine was developed."
'It could happen here'
Dr Mary Ramsey, of the Public Health Laboratory Service, said: "An outbreak of the size that has been described in Ireland could happen here."
She said an outbreak could easily occur among the children who have not been vaccinated when they first go to school.
She said: "We are worried about beginning to see outbreaks from next year or the year after."
Deputy chief medical officer for England Dr Pat Troup said the Department of Health took the concerns of patients about the MMR vaccine very seriously.
But she said research had repeatedly shown that the vaccine was not linked to autism or Crohn's disease.
"We are confident that the vaccine we are offering is safe."
Dr Troup defended the policy of giving children a combined vaccine for measles, mumps and rubella.
She said: "We offer the vaccine in the way that we do because it is the safest way of offering it.
"If we were to split the vaccines we would be leaving children exposed for a period of time when they could contract some of these serious diseases."