The skiing holidays of a large number of Irish holidaymakers in Andorra were marred in recent weeks by an outbreak of winter vomiting disease.
Several visitors to the resort experienced symptoms such as nausea, projectile vomiting and diarrhoea.
The source of their illness is believed to have been the local water supply.
The National Disease Surveillance Centre (NDSC) yesterday urged tourists visiting the ski resort to use only bottled water, even when brushing teeth.
Outbreaks of the same virus in the Republic earlier this year resulted in elective admissions to several hospitals being cancelled and visitor restrictions being imposed.
Dr Paul McKeown, a specialist in public health medicine with the NDSC, said that up to one-third of passengers on three flights from Andorra to Belfast and Dublin had presented with the illness and the Andorran authorities and the Department of Health had been notified.
The upsurge in the incidence of the virus in the Republic this year was worrying, he said. In the first three months of the year there were outbreaks in 36 regions - more than all of last year, when 33 outbreaks were reported.
Latest figures from health boards across the State indicate that there have been more than 2,000 confirmed cases of the virus so far this year, most of them in the eastern region.
The reason for the upsurge was not known, but it might be because people had not been exposed to it in the recent past and had therefore lost their immunity to it. "Or there may be a new variant of the virus. We don't know," he said. "This kind of upsurge is always worrying."
Winter vomiting disease is caused by viruses known as small round-structured viruses (SRSV) or Norwalk-like viruses (NLVs). It is easily spread, but frequent hand-washing with hot water and soap for at least 20 seconds is an effective way of limiting its spread. Although symptoms can be severe, recovery is usually within two to three days without serious or long-term health effects.
The restrictions imposed to curb the spread of the virus have now been lifted at most hospitals.
Restrictions at Waterford Regional Hospital were reimposed last week and all non-urgent elective orthopaedic surgery was cancelled until next Tuesday due to a second outbreak of SRSV. Eight patients have been isolated and 19 staff are off duty as a result, according to a spokeswoman for the South-Eastern Health Board.
The winter vomiting disease outbreak has recurred in University College Hospital, Galway, leading to the postponement of all elective surgery. Visiting has also been restricted.