More than 150 heart surgery patients at a hospital have been warned they may have contracted the dangerous Hepatitis B virus.
Health chiefs at Morriston Hospital, Swansea, issued the alert after a patient who died last month was found to have the virus.
An urgent investigation is under way to find the source of the infection and non-emergency cardiac surgery has been postponed.
All patients who had surgery at the hospital's cardiothoracic unit between March 11 and April 17 could potentially be infected.
They are being offered blood tests to check whether they have contracted the virus.
Managers at ABM University Health Board, which runs the hospital, insisted today that the risk of infection is "low".
The family of the patient who died, understood to be an un-named mother, has since lodged an official complaint.
The woman died after successfully undergoing surgery at the unit and being discharged from hospital after making a good recovery.
She was later diagnosed as having an acute Hepatitis B infection.
The investigation at the hospital has already ruled out staff and family members as a source of the infection.
Blood products used at the hospital have been tested and ruled out as a source, health chiefs said.
Action already taken includes the decision to use more "single-use" surgical instruments to reduce the infection risk.
The hospital's infection control systems within operating theatres is being "reinforced", as is staff training.
A Board spokesman said: "The Health Board was alerted to the problem when a patient who received treatment at the unit during this time, and was discharged as planned after making a good recovery, was later newly diagnosed with an acute Hepatitis B infection. The patient has sadly since died.
"ABM University Health Board and Public Health Wales are carrying out an investigation into this.
"Transmission from staff and family members has been ruled out. It is therefore likely that the virus had been indirectly transmitted from another patient at the cardiothoracic unit, who was known to be Hepatitis B positive.
"The initial investigation has not identified the cause and further investigations are continuing. We are also commissioning an external review."
He said that only cardiothoracic patients at the hospital during the stated period are being contacted. Patients who have not received a letter do not need a blood test.
The GPs of patients involved have been contacted and provided with information and a helpline has been set up for patients who receive the letters.
Hepatitis B is a viral infection spread through blood and other bodily fluids.
It can cause an acute infection which clears up within a few months although 30% of people have no symptoms at all, and most adults make a full recovery.
In rare cases, it can lead to serious liver damage and death, but treatments are available.
Any patients who may have concerns should contact their GP or NHS Direct Wales on 0845 4647 for further information.