The HSE is investigating an ongoing measles outbreak among young people across the State.
Outbreak control teams have been convened in HSE East, South and Midwest.
Nineteen measles cases linked to the outbreak have been identified to date, of which 13 are confirmed and six are probable.
All cases have been identified in the under 30 age group, with most cases occurring in the 15-19 year age group.
The HSE says it believes the source of infection most likely came from another European country where measles outbreaks are occurring.
Most of those who have been infected as a result of this outbreak did not know that they had been in contact with measles.
It is also known that most had not received the MMR (Measles-Mumps-Rubella) vaccine.
Dr Kevin Kelleher said measles is highly infectious and if cases are isolated early, the risk of transmission to vulnerable people decreases.
“The time between exposure to the virus and developing measles rash is normally 14 days [range 7-21 days]. People are infectious from four days before rash starts until four days after,”said Dr Kelleher.
Areas of the country where individuals confirmed with measles have been while infectious in the past month include Dublin, Dingle and Tralee in Co Kerry, and west Limerick.
People who are fully vaccinated with two doses of MMR vaccine are normally protected.
Most at risk
Those most at risk are those who are not fully vaccinated, babies (younger than 12 months so too young to be vaccinated), and those with weakened immune systems if exposed.
Measles symptoms include: high fever, cough, runny nose, red eyes and a red rash that starts on the head and spreads down the body, which starts a few days after the infection takes hold.
The HSE said the best way to protect against measles is to get the MMR vaccine. Any adult born since 1978 should have received two doses to prevent measles.
It said all children should get the MMR vaccine at 12 months of age and the second dose at 4-5 years of age.
The HSE urged parents of any child who missed their scheduled MMR vaccine dose to contact their GP to get the age appropriate dose.
The HSE said if anyone develops measles symptoms, they should stay at home and phone their GP, tell the doctor or nurse that they think they might have measles and stop visitors coming to the house to prevent spreading the infection.