Letters are going out today to thousands of Ottawa area residents, warning them there's a slim chance they could have been exposed to HIV or hepatitis.
Registered letters are being sent to as many as 6,800 people who dealt with a single physician at a "non-hospital" clinic.
Officials say the chance that any one of those receiving a letter may have been exposed to serious illness is remote.
The potential exposure also may have occurred over a 10-year period, raising questions about whether officials will be able to deliver the letters to everyone who could be affected.
The city's medical officer of health hasn't identified the clinic or the procedures involved.
But Dr. Isra Levy says a "lapse" in medical procedure controls could have put people at risk of infection.
Health officials say there is less than a one in three billion chance that someone was infected with HIV, less than a one in 50 million chance of infection with hepatitis C and less than a one in one million chance of hepatitis B infection.
Former patients should start getting letters as early as Tuesday, when a caller information centre is also expected to be up and running.
It's unclear why health officials revealed the exposure to the general public before approaching individuals.