Parents are being urged to get their children vaccinated with the MMR jab after an increase in measles cases at a school in Gwynedd.
Ten children associated with Ysgol Eifionydd secondary school in Porthmadog have been diagnosed so far.
All cases had either had no MMR vaccination, or only one dose - two doses are needed.
A vaccination session for children who have not had both doses will take place at the school on 22 February.
Parents at the school in Porthmadog will receive a letter on Monday about the outbreak.
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.
"Measles is highly contagious and spreads very easily. We would therefore expect to see more cases," he said.
"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."
Dr Whiteside said children usually receive their first dose of the vaccine at 13 months old and the second dose at around three years and four months of age.
Many people who catch measles will have a fever, cough, red eyes and blocked nose and feel generally unwell.
The blotchy rash appears a few days later, beginning on the face and spreading downwards to the rest of the body over several days.
Children should not attend school or nursery for five days after the rash starts, 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," Dr Williams added.
Any parent whose child is suspected of having measles should contact their GP or NHS direct Wales on 0845 4647.