Officials say at least two patients have been infected with the virus, Walesonline reported.
The worker's career has mostly been in Wales, although there were earlier posts in England, Scotland and Northern Ireland.
Aneurin Bevan University Health Board said about 5,500 people will receive letters saying they were in contact with the infected worker, or may have been. The worker was mostly involved in gynecology and obstetrics.
Hepatitis C is a virus that mostly infects the liver. While those infected often have no symptoms, the virus can cause cirrhosis and in rarer cases liver cancer and other serious diseases.
Health officials say the virus can be treated easily once it has been identified.
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