More than 700 patients of a former Gwynedd health care worker diagnosed with hepatitis C called a special hotline for advice on its first day.
The hepatitis C virus is blood-borne
About 5,000 patients have been sent letters with the phone number, and 400 people have now booked blood tests for hepatitis C and B and HIV.
Five of the 47 special clinics set up for blood testing were fully booked.
But the National Public Health Service for Wales has stressed the risk to patients is "very small indeed".
The NPHS discovered that the member of staff, who was understood to have worked at a dental surgery, had hepatitis C in October.
Free testing is being offered to some 5,000 patients who may have concerns.
The confidential hotline for patients to book blood tests was opened on Wednesday, and 717 people called on the first day.
Special blood testing clinics are being held around Gwynedd between 5 June and 17 July.
Dr Sandra Payne, regional director for the NPHS for Wales, said: "Our lines are very busy and I am grateful to patients for the understanding they are showing.
"People are understandably concerned and I would like to emphasise that the risk of patients getting hepatitis C, hepatitis B or HIV is very small indeed."