The Health Service Executive has revealed it is investigating the discovery of Hepatitis B in up to 1,000 patients who were treated in hospitals in Waterford and Wexford.
The patients concerned, who are from all over the south east, attended Waterford Regional Hospital and Wexford General Hospital between January 2005 and February 2006.
The Microbiology Department in Waterford discovered that the patients may have been exposed to the liver disease.
The cases are currently being reviewed by an expert group, established by the executive to advise the South East Hospital Network Incident Team.
"The South East Hospital Group is erring on the side of caution ... as the risk of contracting hepatitis B is very low," the Health Service Executive South said in a statement. "It has recommended that a look-back be carried out to establish if any other inpatient that had a procedure and was in the vicinity of a hepatitis B patient could have been at risk of infection," the statement said.
"The team is currently finalising arrangements for the look-back to take place early next week."
Each patient who was at risk will be individually contacted, the HSE said. They will be asked to attend their GP for a blood test.
The procedures do not relate to surgical or dialysis procedures.
Hepatitis B is a viral illness affecting the liver. Transmission mainly occurs when there is contact between infectious secretions - mainly blood - and cut skin.
The virus can live up to seven days on work surfaces, even dried in drops, which are invisible to the human eye. The incubation period for those who are symptomatic is approximately six weeks to six months, although most commonly it is a two to four month period.
The vast majority of adults recover fully from acute hepatitis B, normally within six months, and remain immune for life from another hepatitis B infection. Some people may be ill for days or weeks and then recover. Others may recover without realising that they have been infected, a small percentage of infected people may develop a more serious illness. The look-back will not affect staff working in the hospitals as they have not undergone procedures The HSE statement added that for the past decade, all frontline hospital staff have been offered vaccination against hepatitis B through the Occupational Health Department.