The measles outbreak concentrated around the Swansea and Neath Port Talbot has spread to parts of west Wales.
Hywel Dda Health Board has urged parents in the area to ensure their children are given two of the three-in-one measles, mumps and rubella vaccine.
So far there have been 23 cases of the illness across Carmarthenshire, Pembrokeshire and Ceredigion so far this year. They say there were five cases in the whole of 2012.
Letters are currently being sent to families in Llanelli, where cases of Measles are up 300% compared to the same period in 2012.
Dr Marion Lyons from Public Health Wales has told ITV News that parents must get their children vaccinated before school trips abroad, or other activities, during the easter holidays.
She said new cases are coming through every day, and extra clinics are being put on.
Letters are being sent out to parents across South Wales.
Dr Meirion Evans, a public health expert at Cardiff University, says there is a backlog of older children who have not had the MMR vaccine, and has urged people to make sure their children are immunised.
He told our Correspondent Richard Morgan that tens of thousands of children are potentially at risk if the outbreak spreads beyond the Swansea area.
Tens of thousands of children are potentially at risk if the measles outbreak spreads beyond South West Wales, according to a public health expert.
Dr Meirion Evans from Cardiff University says he expects hundreds more cases to be confirmed in coming days and weeks.
Dr Richard Roberts, head of Public Health Wales's vaccine preventable disease programme, said the measles outbreak is accelerating and although the "vast majority" of parents have their children vaccinated there is a small minority who have not had the MMR vaccine.
"It is the largest outbreak of measles that we have had for more than 10 years in Wales", he said.
There are now more than 300 cases of measles across South West Wales.
Officials have warned that it is only a matter of time before permanent damage is done, or a child dies.
One mother has told ITV Cymru Wales she became seriously concerned when two of her children fell ill as part of the outbreak.
She's now calling on parents to have their children vaccinated, as Kevin Ashford reports.