An undetectable viral load is a term used to describe the level of HIV in the body when it is so low that it cannot be detected by standard viral load tests. This means that there is not enough of the virus in body fluids to pass on HIV during sex, including oral sex, vaginal sex, and anal sex. Doctors call this "virological suppression" or "having an undetectable viral load". Having an undetectable viral load means that there is less HIV in the body, which keeps the immune system working and prevents illness. It is achieved by taking antiretroviral therapy (ART) or HIV medicine exactly as prescribed. Almost everyone who takes HIV medicine as prescribed can reach an undetectable viral load, usually within one to six months after starting treatment. However, it is important to note that treatment is not a cure, and HIV is still in the body when the viral load is suppressed, even when it is undetectable.

There are important health benefits to having a suppressed or undetectable viral load. People with HIV who know their status, take HIV medicine as prescribed, and get and keep an undetectable viral load can live long and healthy lives. In addition, people with HIV who take HIV medicine as prescribed and get and keep an undetectable viral load will not transmit HIV to their HIV-negative partners through sex. This is sometimes called treatment as prevention or undetectable = untransmittable (U=U) .