Aneurin Bevan University Health Board is leading the so-called 'Look Back Exercise', as the retired healthcare worker infected with Hepatitis C worked mainly at the old Caerphilly District Miners’ Hospital.
It is writing to at least 3,000 obstetric and gynaecology patients this week, and 2,000 more next week, who have possibly, or definitely, been treated by the worker.
The vast majority are in the Aneurin Bevan area, with a small number in Cwm Taf and Betsi Cadwaladr.
Patients will receive an individual letter that will ask them to ring a dedicated helpline to arrange an appointment in a specialist nurse clinic.
Blood tests are being offered in what the health board says is "a precautionary measure."
Dr Gill Richardson, Director of Public Health for Aneurin Bevan University Health Board said, “We know that this will be a worrying time for those patients who will be contacted by letter, but we want to stress that the risk of transmission is low."
“Specialist nurse clinics have been set up and we will do all we can to support patients during this time. As many as one in 250 people carry 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.”