Measles: 189 cases in Swansea, Neath and Port Talbot
28 February 2013 Last updated at 17:34 GMT
About 8,500 children in Neath-Port Talbot and Swansea could be at risk of measles because they have not had their full vaccines.
Health officials have issued the warning because of a serious outbreak in the area.
There have been 190 cases over the past four months with 20 in just the past week. There were 116 in the whole of Wales in the whole of 2012.
Dr Marion Lyons, director of health protection at Public Health Wales, told Gareth Lewis on Good Evening Wales that it is circulating widely.