FAMILIES and patients will have to be on their guard against the winter vomiting bug for up to two months.
The outbreak of the highly infectious virus, which is hitting thousands of victims, is forecast to last another 10 weeks.
Between 1,000 and 5,000 people a week are falling ill with the bug, which is spreading through hospitals, schools and workplaces.
The Health Protection Surveillance Centre (HPSC) said the current strain has been circulating in this country for the past five years.
In St Vincent's Hospital in Dublin, some 92 patients and 70 staff have been hit by the illness and this pattern is being repeated throughout the country.
The virus is capable of spreading from person-to-person, by food and water, via surfaces and through the air.
It can survive for a number of weeks in the environment and on surfaces such as door handles, toilet pulls, TV remote controls, light switches or worktops.
The main symptoms are nausea, vomiting and diarrhoea and usually begin very suddenly. The HPSC says the massive spike in cases may not peak until January.
The week before last, the number of reported cases in hospitals was four times the normal rate while last week it was more than twice the rate.
Outbreaks in some hospitals mean patients who cannot go home for Christmas will only be able to have visitors on a very restricted basis.
St Vincent's says it closed 10 beds to new admissions because so many staff have been laid low but no service has had to be suspended yet.
Visitor restrictions are set to be in place for the festive period, the hospital added.
A number of badly affected hospitals have asked parents not to bring children under 12 years of age on visits.
Dublin's Temple Street Children's Hospital said its visiting restrictions will remain in place for the coming weeks.
The bug, norovirus, is the gastrointestinal equivalent of the common cold.
Up to 5pc of the population can expect to be affected by this virus in years of high activity.
People who contract it can be infectious for 48 hours after they recover.
The most important steps to take in reducing its spread in hospitals are to immediately clean any contaminated areas, to exercise good hand hygiene and to segregate those people who are ill.