A Welsh assembly member has criticised health chiefs for not announcing sooner a dental worker had hepatitis C.
Up to 5,500 dental patients need to be traced in Gwynedd
It emerged last Friday that up to 5,500 Gwynedd patients will be screened after the person was diagnosed in a case that has been worked upon since last year.
Caernarfon AM Alun Ffred Jones said it appeared to be "a very long time" before alerting the public.
A National Public Health Service spokesman said identifying those at risk was a "very meticulous" process.
Dr Brendon Mason denied there had been a delay in tracing up to 5,500 patients.
Dr Mason said that as soon as the worker was found to be carrying the virus in October last year a "risk assessment" had been carried out and advice taken from the UK advisory panel on blood-borne viruses.
Health staff were now going through up to 30 years of medical records covering the worker's career to identify anyone who might have been infected, said Dr Mason.
"We understand that people are worried, we want to write to patients when the counselling and blood tests are ready - we have no desire to be secretive," Dr Mason added.
He said a "process was in place" to deal with the situation and the risk to patients was "very low indeed".
Identifying up to 5,500 people and tracing them would take time, he said.
Of the patients who could have come into contact with the virus from the worker some might have picked the virus up elsewhere, said Dr Mason.
If someone was found to have the virus during screening there was no guarantee that it had been caught from the worker, he added.