North Wales' health board says the healthcare professional infected with Hepatitis C worked briefly at Wrexham Maelor Hospital (known then as the Maelor General Hospital) in May and June 1978.
Betsi Cadwaladr University Health Board says the risk of passing on the virus during a healthcare procedure is low, and could only happen if the worker suffered an injury causing them to bleed while treating the patient.
It says it has been reviewing its records, and obstetric and gynaecology patients from that time have been offered advice a blood test as a precautionary measure. Specialist clinic sessions will be held at Wrexham Maelor Hospital.
Andrew Jones, Director of Public Health for the Betsi Cadwaladr University Health Board, said: “I know that this news will cause some concern for patients who were seen in Wrexham at around that time. However I want to stress that the risk of transmission is low."
"Even so, it is important that we contact patients who were treated by this person and offer them support and the opportunity of a blood test. This will allow us to give reassurance that all is well or, if we do identify a person who is carrying the virus, ensure they get advice and treatment."
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