Public Health Wales are urging parents to get their children vaccinated, as the number of measles cases reaches 316.
According to new figures, there have been 64 new cases reported in the last week.
The disease has now spread to children in 111 secondary and primary schools, nurseries and playgroups.
The majority of cases are in the Swansea area but cases are increasingly being reported across the Mid and West Wales region.
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.