The number of cases of mumps in Wales has more than doubled during the last year, the latest figures have revealed.
Many children missed out on the MMR vaccine
There were more than 3,000 reported cases of mumps in Wales in 2005. In 2001 there were just 72 cases.
The sharp rise - in line with an increase across the UK - has prompted the assembly government to urge people aged 11-25 to receive the MMR vaccine.
Health officials said mumps could be a "serious illness", but was largely preventable by the vaccine.
Wrexham GP Peter Saul said he had noticed and increasing number of cases coming into his surgery, particularly over Christmas.
He said: "It's the older teenagers and people in their early 20s who seem to be the ones who are catching it."
Significant outbreaks of mumps have been reported in schools, colleges and universities across the UK.
Dr Saul told BBC Radio Wales the increase in mumps cases was a historical problem.
"In the 80s the MMR vaccine was introduced and at that time children just had the one shot round about the age of one or just over," he said.
"A lot of children were protected, but a few years later the health experts found that it wasn't giving them full protection and GPs like me were asked to give an additional booster dose when the children were four or five.
"But before that happened a number of years of children had slipped by and it seems to be it's these children, who are grownup now, who are at risk of getting the mumps."
A large group of children have not had any vaccine, because their parents did not agree with having MMR, he added.
Symptoms of mumps include swollen glands, fever and a general feeling of being unwell.
The disease can also can give you inflammation of some organs in the abdomen.
Mumps can infect the central nervous system and can be fatal - but Dr Saul said such cases were very rare.
He urged older teenagers and people in their 20s who had not received the vaccine to go to their GP for the booster injection.
Richard Roberts, from the National Public Health Service for Wales, made a similar plea for people to be vaccinated.
"There has been a very dramatic increase in the incidence of mumps across the UK in general, and Wales is no exception," he said.
"Mumps can be a serious illness resulting in hospitalisation and complications, but it is largely preventable through the MMR immunisation, which protects over 90% of those who receive one dose and 99% of those who receive two doses of the vaccine.