The Centre for Disease Control, CDC, has declared that HIV patients cannot transmit the virus to sexual partners if they have suppressed their viral load with medication.
Until now, the agency has refused to say for certain that people who religiously take HIV drugs are not a danger to society.
But for years, activists and researchers have been campaigning for that to change since every clinical trial and cohort study ever conducted shows those with an 'undetectable' virus have a zero percent risk of transmission.
The move, announced in a letter last week to mark National Gay Men's HIV/AIDS Awareness Day, has been hailed as a 'breakthrough against stigma and for HIV prevention' by the HIV community.
It means the Centers for Disease Control and Prevention has joined scores of public health agencies and more than 400 organizations from nearly 60 countries in affirming that people who take medication are not dangerous to sexual partners.
A person with HIV becomes 'undetectable' when treatment suppresses the virus to a level so low in their blood that it cannot be detected by measurements.
Scores of studies on more than 40,000 people have shown that if a person is undetectable and stays on treatment, they cannot pass HIV on to a partner.
The strength of this association first became clear a few years ago, and gradually health officials have been acknowledging the results.
Dr Anthony Fauci, head of the HIV/AIDS division at the National Institutes of Health, was the first high-level figure to back the statement that 'U=U' (undetectable equals untransmittable'.
Last year, the CDC came close, stating that the risk of transmission was 'negligible'.
Finally, on September 27, Dr Eugene McCray and Dr Jonathan Mermin, the co-directors of the CDC's HIV/AIDS division, went a step further.
Their letter read: 'People who take ART [anti-retroviral therapy] daily as prescribed and achieve and maintain an undetectable viral load have effectively no risk of sexually transmitting the virus to an HIV-negative partner.'
Speaking to Daily Mail Online, Dr Fauci emphasized the gravity of their statement.
'They have never said undetectable equals untransmittable.
They have explained the data and acknowledged findings of cohort studies and clinical trials, but they have never come out and said it the way the gay community wanted to hear it. That is huge.
'When you're dealing with biology, people are really concerned with the exceptions. But when you make recommendations for people's health, you have to go with the overwhelming data.
'Maybe once in a while there might be an exception but that is always down to a mistake - the person thinks they're undetectable when they're not, or they are not taking their medication as they should.
'If you are truly undetectable there is no risk of transmission.'
Previously, public health officials have cited concerns that we need to explore more precise HIV testing methods to detect hidden traces of the virus. Trials are under way to explore how the virus could linger in sperm, for example.
While those concerns are valid, the CDC and NIH have concluded that even though someone with HIV could have hidden reservoirs of the virus, there is conclusive evidence that those traces are not viable and cannot be transmitted.
Dr Fauci said the CDC's statement will have a two-fold positive effect: dispelling societal stigma and personal stigma.
'No one should ever under-estimate the concern about social stigma - the pressure people put on themselves,' Dr Fauci explained.
'There are people out there who have guilt and fear about getting tested positive. They are afraid of the stigma of being positive, so they may not get tested.
'But now, if you know that by taking medication you are dangerous to nobody, that does have a profound impact psychologically. People won't be afraid to get tested and start medication so they can suppress their viral load.'
