More than 5,000 patients of a Gwynedd health care worker are to be screened for HIV as well as hepatitis B and C.
The hepatitis C virus is blood-borne
It follows the member of staff, believed to be at a dental surgery, being diagnosed with hepatitis C.
The National Public Health Service (NPHS) for Wales said it was also offering screening for HIV and hepatitis B.
It follows patient concerns about the standard of the health care worker's infection control procedures.
The worker was diagnosed with hepatitis C last autumn and on Wednesday the NPHS stressed the person had only that infection.
'Very small' risk
Doctor Sandra Payne, regional director for the NPHS North Wales, said: "The contact programme is precautionary.
"The risk of a healthcare worker passing on the hepatitis C virus is very low indeed. It can only happen if the healthcare worker's blood gets into the patient's bloodstream
"We have no evidence to suggest that any individual has suffered ill health as a consequence of having care provided by the healthcare worker.
"The letters to patients will arrive in the next couple of days and I hope that people will use the advisory and support services we have set up. People who do not receive a letter have no reason for concern."
She added: "I do want to emphasise that the risk of patients getting hepatitis C, hepatitis B or HIV is very small indeed."
The NPHS has checked the details of more than 5,000 patients and examined records going back up to 30 years in a bid to identify those who are to be offered screening.
The tests will be offered from 5 June to 17 July at 47 special clinics set up for blood testing.
Mr Ifan Williams, 70, received a letter on Wednesday. He said: "I feel really angry because they are putting patient confidentiality before public health."
He said he was also concerned because the symptoms described on the pamphlet that came with his letter are identical to ones he has experienced as a diabetic.
"I don't know whether the symptoms are because I am diabetic or because I have hepatitis C," he said.
The NPHS has been working on the case since last year and an incident management team has also been set up to look into the case.
The member of staff concerned stopped working last October, when the NPHS was notified of their diagnosis.
Hepatitis C means swelling or inflammation of the liver. The virus is blood borne and is spread when blood of an infected person is spread into the bloodstream of another.
The infection affects different people in different ways, with many experiencing no symptoms at all while others experience extreme tiredness and can feel very unwell.
Reported symptoms include fatigue, weight loss, nausea, 'flu-like symptoms, problems concentrating, abdominal pain and jaundice.
It is estimated that around 15-20% of infected people clear their infections naturally within the first six months of infection.
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