Ireland is on high alert following a serious outbreak of measles, with more than 20 people affected.
The latest outbreak, which hit Kerry in mid-April, has seen 22 people contracting the virus which in rare cases affects the lungs and brain.
Seven more people are undergoing tests to find out if they have caught measles. The outbreak has already led to several hospitalisations for the potentially life-threatening illness.
Those with the virus range in age from infants and teenagers to people in their 20s.
Nineteen of the 22 confirmed cases have had to be admitted to Kerry University Hospital for complications and most have now recovered.
HSE public health specialist Dr Fiona Ryan said measles is all but eradicated in Ireland, but the highly-contagious illness was spread after people who were infected abroad returned here. Around 30 people nationwide have been diagnosed this year compared with five last year and 33 in 2014.
"The way it comes into Ireland now is that somebody brings it in. Somebody who was infected travelled back to Kerry and this causes it to circulate in the community," said Dr Ryan.
The HSE is not saying what country the Kerry outbreak originated in.
It has listed areas where different people, who turned out to have measles, passed through including Dublin Airport in mid-May.
An infected person was also on a Ryanair flight from Croatia to Dublin and another was in Limerick city while others were in Ballina, Co Mayo, and Enniscrone, Co Sligo.
The MMR (measles, mumps and rubella) vaccine is given in two doses, at 12 months and at age four or five years.
The vaccine take-up is high, but it is still not enough to ensure "herd immunity".