Up to 5,500 patients in Gwynedd are to be screened for hepatitis C after a health care worker was diagnosed with the virus.
The hepatitis C virus is blood-borne
Unconfirmed reports suggest the case was diagnosed at a dental surgery.
The National Public Health Service for Wales (NPHS) said the risk was small and all at risk would be contacted as soon as a list was completed.
The public health service has been working on the case since last year and an incident team has also been set up.
On Friday, NPHS issued a statement which said; "The team has assessed the risk to patients and is preparing a comprehensive communications strategy to contact patients who have been treated by the healthcare worker.
It added counselling and screening would be offered.
"The risk of patients contacting hepatitis C is very small and the contact programme is being taken on a precautionary basis," the statement said.
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.
Hepatitis C infection affects different people in different ways with many experience 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 6 months of infection.
Charles Gore, chief executive of the Hepatitis C Trust, said: "The danger is certainly very slight, but assuming this is just an infected worker, there's no reason why in a dental surgery their blood should be spilt onto your blood and get into your system.
"Nonetheless, it's a very sensible precaution to screen everyone".
He added: "In a worst case scenario, if someone was infected, there is now very effective treatment available, particularly effective if the diagnosis is early and is caught in the first six months.
"Although this is a very serious disease, there's a lot that can be done about it and in this case, the risk is going to be very small".
A hepatitis C helpline is on 0845 600 6086. The helpline is open between 0700-1600 BST over the weekend. From Monday to Friday, the line will be open between 0700 and 2000 BST.