Dr. Eilish Cleary, the chief medical officer of health, said a New Brunswick man recently tested positive for the virus and officials are now attempting to contact his sexual partners from the last two years.
"In the course of our investigation we became aware that this individual had many contacts and had met many of them on an anonymous dating site," Cleary told a news conference, identifying the website as manhunt.net.
She said the man and the website have been helpful in trying to find his sexual partners, though it is a difficult process because people typically log on using aliases rather than their real names.
"People can have moved and they can have other contacts in the meantime, so our message is a general message of awareness for everybody," Cleary said.
"If you are using those sites to meet people, you really are placing yourself at higher risk and so you should take precautions. Wear a condom."
Her office issued a statement encouraging people who had unprotected sex with those they don't know to get tested for sexually transmitted diseases.
The Health Department is not identifying the man or giving his age. Cleary said health officials have no reason to believe he put people deliberately at risk.
She said most of the man's sexual encounters occurred in New Brunswick. Her department is also following up with other jurisdictions, but she did not say where those were.
Last year, New Brunswick reported an outbreak of syphilis that saw the number of cases rise tenfold within three years. Cleary said Monday the number of syphilis cases is on the decline.
The Public Health Agency of Canada estimates there were 3,175 new HIV infections in the country in 2011.