U=U (Undetectable = Untransmittable) is a global campaign to spread the awareness about the scientific FACT that a person with an undetectable HIV viral load can’t transmit the virus to their sexual partners.

What does ‘undetectable’ mean? People who are taking their antiretroviral treatment (ART) for HIV, and the treatment has worked so effectively that the virus is reduced to such a low level that it's classed as undetectable. It is sometimes referred to as an Undetectable Viral load, meaning the level of the virus present is someone's bloodstream is very low. 

What does ‘untransmittable’ mean? When someone is on effective ART and has an undetectable viral load, it is impossible for them to spread HIV sexually. Therefore, even through they live with HIV, it cannot be spread sexually to someone else.