Test results are due this evening from residents at a private nursing home in Co Donegal where six people died from a respiratory illness in the last 12 days.
Twenty-seven other residents at Nazareth House in Fahan, Buncrana are ill with symptoms, although none of these are in a critical condition.
Investigations are being carried out by the Health Service Executive and officials from the Health Information and Quality Authority.
The Health Protection Surveillance Centre says it looks like the deaths could have been caused by a viral illness or a strain of flu.
Dr Darina O’Flanagan, director of the Health Protection Surveillance Centre, said it is suspected the deaths were as a result of a virus.
This could be a particular strain of influenza prevalent this year, which was more likely to affect older people.
“If this is flu, there is a type of virus this year that is more associated with the elderly,” she said.
Anti-viral treatments have been given to all of the residents.
Microbiologist specialists were part of a team of health officials also sent to the home.
A quarantine zone has been thrown up around the care home, which is off the main road between Buncrana and Derry.
All of the residents, both men and women, are believed to be over the age of 80 years.
A full-scale investigation was launched after health chiefs were notified of the deaths which occurred between March 22 and April 2.
The HSE said it was notified last night.
Health Minister James Reilly and Older People’s Minister Kathleen Lynch issued a statement saying they had learned with regret of the deaths and are being kept fully informed.
Samples have also been sent to the National Viral Reference Laboratory for determination.
The ministers both expressed their condolences to the families of the deceased residents.