Health officials say 64 new measles cases have been reported in the Swansea area in the last week, taking the outbreak to 316 with 42 hospitalised.
In total, 111 secondary and primary schools and nurseries have now been affected.
Public Health Wales, urging MMR vaccinations, also warned of increasing cases across mid and west Wales.
It says it is only a matter of time before a child is left with serious and permanent complications or even dies.
"We cannot emphasise enough that measles is an illness that can kill, or leave people with permanent complications including severe brain damage," said Dr Marion Lyons, director of health protection for Public Health Wales.
"The only protection is the safe and highly effective MMR vaccine.
"We are urging parents of unvaccinated children to make immediate arrangements with their GP for their children to receive the MMR jab, to prevent further spread of the disease and further misery."
Typical symptoms of measles include fever, cough, conjunctivitis and a rash.
Complications are quite common even in healthy people, and around 20% of reported measles cases experience one or more complication.
These can include ear infections, vomiting and diarrhoea, pneumonia, meningitis and serious eye disorders.
Dr Lyons added that children who have not been fully immunised face a life-long risk of catching measles but said one dose of MMR would protect 90% of children very quickly and two doses protects 99%.
She said: "The current outbreak will almost certainly continue into the summer months with potentially serious impacts for children sitting important exams, such as GCSE's and 'A' levels.
"Holiday plans may well also suffer if the disease continues to spread."