Maintain an undetectable viral load

People with HIV should take medicine to treat HIV as soon as possible. HIV medicine is called antiretroviral therapy (ART). If taken as prescribed, HIV medicine reduces the amount of HIV in the body to a very low level, which keeps the immune system working and prevents illness. This is called viral suppression. HIV medicine can even make the viral load so low that laboratory test cannot detect it. This is called an undetectable viral load—defined as having less than 20 copies of HIV per milliliter of blood.
Being undetectable means people with HIV have effectively no risk of transmitting HIV to an HIV-negative partner through sex (oral, anal and vaginal), hence untransmittable (Undetectable = Untransmittable, U=U).
By taking medicine daily, as prescribed, most people can get an undetectable viral load within 6 months.

*The above approaches are only for HIV prevention but not other STIs.
Undetectable = Untransmittable (U=U) is proven by several studies. In 2016, the PARTNER study from Europe found that the chance of HIV transmission where one partner had a UVL is negligible. In fact, there were zero partner-transmissions recorded in the study despite approximately 22,000 acts of condomless sex and not using PrEP by MSM couples. Additionally, PARTNER2 – the extension study of PARTNER that aimed to gather data pertaining to mixed-HIV-status gay male couples – found that after an additional 54,000 acts of condomless sex there were zero transmissions.
Another study led by the Kirby Institute, the Opposites Attract study, which focused solely on gay and other men who have sex with men from Australia, Brazil and Thailand, confirmed that HIV positive men who are on treatment and have an undetectable viral load do not transmit the virus to their partners. Thus, receiving proper treatment as early as possible can effectively manage the body condition and prevent transmission of HIV.