Undetectable = Untransmittable
It is a fact.
A person living with HIV and is being effectively treated CANNOT transmit the virus.
It's probably the most positive message a person living with HIV can hear. It reduces the stigma around HIV, provides additional incentives to stay in treatment, and stops the transmission of the virus to one's sexual partners.
If we all knew this simple and yet hopeful message, it would finally end the stigma around HIV, while, at the same time, we could forever end the transmission of the virus.