The HSE says an outbreak of measles in Dublin has spread.
Last month, two people were diagnosed with the virus after returning from mainland Europe.
Health bosses fear four more people confirmed to have measles may have come into contact with the first two.
People are being advised to get the MMR jab, and to phone their GP if they suspect they have any symptoms.
Dr Helena Murray, Specialist in Public Health Medicine said “Measles can be a serious illness and is highly infectious."
"The best protection is to be fully vaccinated with two doses of MMR (Measles, Mumps, and Rubella) vaccine", she warned.
Measles symptoms include:
- High fever
- Runny nose
- Red eyes
- Red rash that starts on head and spread down the body - this normally starts a few days after onset of illness. The rash consists of flat red or brown blotches, which can flow into each other. It lasts about 4-7 days
- Vomiting, diarrhoea and tummy pain may also happen.