The number of Respiratory Syncytial Virus (RSV) cases in Ireland has reached a new and "unprecedented" record high.
731 new cases were reported to the Health Protection Surveillance Centre in the seven days up to the end of last week.
About 65% of those 731 cases were in children aged between zero and four, while the next most affected group were aged over 65.
290 people were hospitalised with RSV in the past week, with those affected predominantly children under four, with some older people also affected.
RSV is a common winter infection. Symptoms often include a fever, cough or runny nose.
The latest figure broke the previous record set in recent weeks, where 650 new cases were reported in one week.
The Director of the HPSC said that this surge in the virus has hit six weeks earlier than normal.
Dr Greg Martin described the latest figures as unprecedented and said that there would be "many more" cases in the community that have not been officially notified.
He said that in previous years, the peak figure would be about half what was reported just last week, so Ireland is now experiencing "substantially more" RSV cases than would be expected in any given year.
Dr Martin said the reason for this is likely linked to the fact that during the Covid-19 pandemic, children had less exposure to RSV and therefore have less natural immunity now.
He said that it is not known if RSV has hit its peak for this season yet.
"We are hoping that it is as high as it will go, and we'll see the numbers coming down from next week, but we don't know that for a fact yet," he said.
Dr Martin said the fact that 290 people were hospitalised with the virus last week underlines how serious an illness it can be for some people, while for most it is just like a common cold.
He said that efforts must be made to protect the most vulnerable and advised people to take the offer of a flu vaccine.
He said that public health measures like good cough etiquette, hand hygiene and staying at home when sick will also protect others.