The HSE is advising parents to keep their children home from school if they show signs of being unwell.
It says it's concerned at the spread of a rare bacterial infection known as Group A streptococcal.
Symptoms typically include runny or blocked nose, mild fever, cough or tiredness.
Dr. Maureen Boland of the HSE, says infected children are more likely to spread harmful viruses:
"It's important for a child who is particularly unwell with fever, cough or sore throat to stay at home and to minded at home until these symptoms have finished and are cleared - because really children with these symptoms are much more likely to spread infections - this could be flu or RSV, or indeed a bacterial infection".
