Six hospitals across the State have now placed restrictions on visitors in an attempt to prevent the spread of the winter vomiting bug.
Waterford Regional Hospital and Cork University Hospital are among the worst affected and have placed a complete ban on visitors. Only people with critically ill relatives in these hospitals are permitted to visit.
A total ban on visitors is also in place at St Mary's Orthopaedic Hospital in Cork and at St Clare's ward of St Finbarr's Hospital, also in Cork.
Meanwhile, the public has been advised not to visit the Mid Western Regional Hospital in Limerick unless it is absolutely necessary. A spokesman said two cases of the winter vomiting bug were being treated there.
At Mullingar General Hospital, visits to the paediatric unit are restricted. A spokeswoman said one suspected case of winter vomiting was being treated.
The virus is easily spread from person to person and can survive in the environment for days, making it difficult for hospitals to contain.
People infected are contagious from the moment they begin feeling ill until two to three days after recovery. Some people may be contagious for as long as two weeks after recovery.
Symptoms include nausea, vomiting and watery diarrhoea, and while most people make a full recovery within one to two days, some people (usually the very young or elderly) may become very dehydrated and require hospital treatment.
Yesterday Waterford Regional Hospital said it had placed a complete ban on visitors until further notice, due to an outbreak of the virus.
"The only exceptions to the visiting ban is the family of critically ill patients who are requested to ring the ward before visiting the hospital," it said in a statement.
It stressed, however, that the chemotherapy and outpatients departments remained open as they were separate from the main wards in the hospital.
The radiotherapy and chemotherapy day units and the outpatients department at Cork University Hospital also remain open.
"Patient care is our priority. We would appeal to the public who may have experienced vomiting and or diarrhoea to avoid visiting any hospital or nursing home until they are symptom-free for 72 hours [ three days]," said the general manager of Waterford Regional Hospital, Patricia Sullivan.
The Health Service Executive (HSE) said people affected by the virus should drink plenty of fluids and maintain strict hygiene. "If symptoms persist, contact your GP by telephone and advise him/her of your condition before going to the surgery so as to limit the spread of the virus," it advised.
There are usually several outbreaks of the winter vomiting bug in hospitals across the State each year as well as many cases in the community.
The national Health Protection Surveillance Centre (HPSC) says outbreaks are reported frequently anywhere that large numbers of people congregate for periods of several days such as hospitals, schools, residential and nursing homes, cruise ships and hotels. This is because these places provide an ideal environment for the spread of the disease.
Guidelines on the management of outbreaks of the infection in healthcare settings are published on the HPSC's website.