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.
Fresh measles cases have been reported in Powys and Cardiff. A public health expert warned tens of thousands of children could be at risk.
Public Health Wales has information and advice on measles - a highly-infectious viral illness which can have life-threatening complications.