Canada's top court said on Friday that failing to tell a sexual partner you have HIV is only sexual assault if there is "a realistic possibility" of transmitting the virus that causes AIDS.

In a unanimous decision, the Supreme Court of Canada rejected the government's argument that everyone who has HIV should be required to disclose that condition to all sexual partners in any circumstance.

Several groups involved in the case, including the Canadian HIV/AIDS Legal Network and the Canadian AIDS Society, took issue with the court's standard of a "realistic possibility," saying it is too severe, and calling the decision "a major step backwards for public health and human rights."

The HIV groups said in a release that the risk of spreading HIV is made negligible just by using condoms, and that the court's decision "blatantly ignores solid science."

"What you're talking about here is a vulnerable, marginalized group of people who are going to be forced to go around volunteering to anyone with whom they're going to have sexual contact, that they belong to that vulnerable, marginalized group," Michael Feder, a lawyer for the British Columbia Civil Liberties Association, told the CBC.

In 1998, the court found that not telling a partner about one's HIV status was a form of aggravated sexual assault if there was "significant risk of bodily harm." On Friday, the court clarified what might constitute a significant risk.

It said someone with a low viral load who uses a condom has not put their partner at significant risk. Viral load measures the severity of HIV infection, and some treatments reduce the level, making transmission less likely.

"As noted by the court of appeal, the transmissibility of HIV is proportional to the viral load, i.e. the quantity of HIV copies in the blood," the decision said, according to City News. "The viral load of an untreated HIV patient ranges from 10,000 copies to a few million copies per millilitre.

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"When a patient undergoes anti-retroviral treatment, the viral load shrinks rapidly to less than 1,500 copies per millilitre (low viral load), and can even be brought down to less than 50 copies per millilitre (undetectable viral load) over a longer period of time. This appears to be scientifically accepted at this point, on the evidence in this case," the decision said.

The court noted that standards might change in future cases since its decision does not preclude the law from "adapting to future advances in treatment" or risk factors it had not directly considered.

It said that risk of transmission, rather than actual transmission, is not a crime in many other countries, and that this "sounds a note of caution against extending the criminal law beyond its appropriate reach in this complex and emerging area of law."

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Forcing everyone to disclose their HIV status would risk sending people who have not put anyone at risk to prison, the court said.

Two cases were considered by the court, the first one involving a man who had sex with nine women, including a 12-year-old girl, after being diagnosed with HIV in 2004, CBC reported. Clato Mabior didn't tell the women he was HIV positive.

None of the women contracted the disease, City News said. The Supreme Court set aside one of Mabior's convictions because in that respective situation he used a condom and had a low viral load. In Mabior's case, this decision has little consequence, since he was deported to South Sudan in February, City News said.

In the second case, a Quebec woman who had unprotected sex with her partner in 2000 did not tell him she was HIV-positive.

According to City News, the woman had been taking anti-retroviral drugs after she was diagnosed with HIV nine years earlier.

"Here low — indeed undetectable — viral load was established," the Supreme Court concluded, according to City News.

The court affirmed an earlier appeal court decision to strike down the woman's conviction for aggravated assault and sexual assault.

Reuters contributed to this report.

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