More than 560 dental patients in one surgery have been told to have HIV tests because the equipment is not properly cleaned.
They got letters saying they were in danger after the hygienist failed to sterilize the scaler.
But it took three months for health professionals to work after being informed by Hoddesdon.
One patient said, "You get a letter telling you to test HIV, and that's scary. I can say that the risk is as small as they want – but the truth is they do not really know.
The ultrasound scaler was cleaned only twice a day when it was to be sterilized after each use
"The fact that they had three months to warn them is all disgusting. How many more people could be infected at that time?
The ultrasound scaler was cleaned only twice a day, when it was to be sterilized after each use.
The mistake came to light in January when Hoddesdon, Herts, staffed the staff, realized that the scaler did not put it in the sterilizer as often as it should.
A self-employed hygienist from London is no longer used in practice, but can continue to work with supervision.
It is believed to have no infection, but it is not revealed whether any patient who treated it.
However, health professionals say that 563 patients may be involved in the infected blood.
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Dr. Jorg Hoffmann of Public Health England East said: "The risk of hepatitis B, hepatitis C or HIV infection is extremely low and testing is a precautionary measure."
Dr. Vishaal Shah of Dentality Hoddesdon insisted: "We always put patients first."
Because it is not treated properly because doctors do not realize that there are four subtypes of deadly immune response, American researchers say.