Thousands of people across the UK are being contacted after a healthcare worker infected two patients with the Hepatitis C virus.
A major alert is under way after it was discovered that a retired obstetrics and gynaecology worker unknowingly had the virus while employed by the NHS for decades.
It is known the healthcare worker infected two patients with the virus while working at a hospital in Wales, Public Health England said.
At least 3,000 former patients are being contacted by letter informing them of the risk and a series of confidential helplines and a support service have been set up.
Several hundred patients in other areas of Wales who may have come into contact with the health worker are also being contacted.
In England around 400 former patients are being contacted, while urgent steps are being taken to check historic patient records in Scotland and Northern Ireland.
"It has recently come to light that the healthcare worker is known to have transmitted the virus to two patients while working at a hospital in Wales from 1984 until they ceased clinical practice in 2002," a PHE spokesman said.
"A 'lookback' patient notification exercise involving the review at least 3,000 former hospital patients' notes and records from the Caerphilly District Miners Hospital has been announced by the Aneurin Bevan Health Board today.
"Around 200 former hospital patients from two other hospitals in Wales where the healthcare worker practised for a short time are also being contacted.
"Those patients identified as exposed or possibly exposed to Hepatitis C are being sent individual letters and asked to call a special confidential helpline, inviting them to attend a hospital clinic or, if they have moved away from the area, their GP for a blood test."
He added: "Effective treatments are available for Hepatitis C and further information and advice will also be provided to anyone who needs it.
"There is only a small chance that a patient might acquire Hepatitis C virus infection through surgical contact with an infected healthcare worker.
"The risk is very low as this can only occur if the healthcare worker is infectious and leads or assists in an operation or procedure on the patient. However, even in such circumstances transmission is very rare."
The healthcare worker is known to have worked at other hospitals across the UK prior to working in Wales, including 11 hospitals in England from 1975 to 1983.
"Similar lookback exercises are taking place in parallel across all the hospitals in England, Scotland and Northern Ireland where the healthcare worker practised," the spokesman said.
"Less than 400 women in England have so far been identified as having definitely or possibly had operations conducted by the affected healthcare worker.
"They will be contacted directly by letter inviting them to call a confidential helpline, to discuss whether they would like to have a blood test arranged at their GP practice."
As it has been almost 30 years since the individual worked in hospitals in England, records of women who may be at risk are in some cases incomplete, for example if the hospital has been renamed or patients have moved around the country, PHE said.
In England, the helpline and support service (0800 121 4400) will also be in place from Thursday , for any woman who is concerned because they had an obstetric or gynaecological operation, or they gave birth, at one of the 11 hospitals during the specified periods.
Dr Paul Cosford, medical director at PHE, said: "We have worked hard to identify women in England who might have been at risk of contracting infection with Hepatitis C from this healthcare worker and are contacting them to offer advice and a blood test for Hepatitis C, should they wish to have one.
"I want to emphasise that the risk of infection is very small and that we are offering them testing purely as a precaution.
"Around one in 250 adults in England have chronic Hepatitis C infection and it does not automatically lead to health problems.
"Treatment can help clear the infection in up to 80% of cases, which is why it's important to identify anyone who may be at risk of having been infected so treatment can be started if necessary."
The healthcare worker in question was infected with Hepatitis C but, in kind with most cases, suffered no symptoms and was unaware of the infection.
As soon as the risk of infection was recognised, and a transmission was confirmed, a process of tracing their occupational history began, PHE said.
Since 2007, all healthcare workers who are new to the NHS are tested for Hepatitis C by their employing Trust, including anyone performing certain procedures (known as exposure prone procedures).
Existing NHS healthcare workers performing exposure prone procedures for the first time are also tested for Hepatitis C.
Hepatitis C is a virus which can lead to inflammation of the liver and cause chronic liver disease and, very rarely if untreated, cancer of the liver.
Most people are unaware of their infection because the liver can still operate when damaged and the virus does not cause any symptoms.
It is often only when the liver becomes seriously damaged and symptoms occur that people visit their GP. Antiviral therapies can clear the virus in many cases.
The virus is transmitted through blood-to-blood contact, and very rarely through sexual intercourse. Infection is not possible through social contact, kissing or sharing food and drink.
In the UK, sharing of equipment by intravenous drug users is the commonest form of infection with hepatitis C. Testing first became available in 1991.
Around 10,000 new hepatitis C diagnoses are made in England each year and around 160,000 adults in England are estimated to be living with a chronic infection.
The lookback exercises across the UK are being carried with advice from the UK Advisory Panel for Healthcare Workers infected with Bloodborne Viruses.