Fifteen cases of measles have been confirmed in Co Kerry, in an outbreak traced to April when an infectious person travelled on a flight from Dublin to the county.
Kerry University Hospital, which had not seen a measles case in many years, is asking parents visiting the hospital not to bring children unless there are exceptional circumstances.
Those with suspected measles, a highly contagious disease, should also not attend doctor's surgeries and should instead arrange a home visit.
The 15 cases were confirmed by GPs and public health nurses and the level is now technically considered "an outbreak".
However, there may also be milder cases, which have not been confirmed, said KUH manager Maria Godley.
Dr Martin Boyd, consultant at the emergency department, said measles was difficult to spot until the rash appeared and the most contagious period was four days before the rash, and four days after its appearance.
A sore throat, head cold, runny nose, conjunctivitis, cough and fever are among the symptoms.
"Measles is an extremely contagious disease," Dr Boyd said.
It can lead to brain damage and pneumonia from measles is a serious complication for adults.
There were, however, no serious complications in the 15 cases recorded so far in Kerry.
Those who had got the two sets of MMR vaccinations, administered normally at 12 months and four years of age, were 99% protected against the virus, Dr Boyd added.
