Health chiefs have repeated their warning for children to be vaccinated with the MMR jab after 19 cases of measles among young people in Gwynedd.
Seventeen children associated with Ysgol Eifionydd secondary school in Porthmadog have now been diagnosed, as well as two close family members.
All cases had either had no MMR vaccination, or only one dose - two doses are needed.
A vaccination session has been held at two schools in the area this week.
Parents at the school in Porthmadog received a letter on Monday about the outbreak after Public Health Wales revealed an outbreak at the school.
Dr Chris Whiteside, consultant in communicable disease control for Public Health Wales, said many of the cases under investigation had not received their scheduled MMR vaccinations.
He said: "I therefore urge parents in north Wales who have not arranged immunisation for their children to act immediately to get them vaccinated.
"Measles is highly contagious and spreads very easily. We would therefore expect to see more cases.
"As long as there are children who have not had their two MMR vaccinations, there is the potential for people to become ill with measles.
"There is then a danger for the disease to spread to unvaccinated friends, family or others who cannot be vaccinated due to existing health problems and who are therefore highly vulnerable to measles infection."
Dr Whiteside said health chiefs were not aware of any links between Porthmadog and a current measles outbreak of measles in Merseyside.
He said children usually received their first dose of the MMR vaccine at 13 months old and the second dose at about three years and four months.
He said: "The only way to prevent outbreaks of measles is to ensure that at least 95 per cent of children in Wales have received two doses of the MMR vaccine.
"Although our uptake figures have improved, we still have some way to go before we can guarantee the safety of Welsh children from measles."
Any parent whose child is suspected of having measles can contact their GP or NHS direct Wales on 0845 4647.