Undetectable = Untransmittable (U=U) is a message used in HIV campaigns that means if someone has an undetectable viral load, they cannot sexually transmit HIV to others. HIV medication, also known as antiretroviral therapy (ART), can decrease the amount of HIV present in a persons blood, or "HIV viral load", to be too low to measure, which is called being undetectable. People with HIV who take their medication as prescribed and get and keep an undetectable viral load will not transmit HIV to their HIV-negative partners through sex. This prevention method is estimated to be 100% effective as long as the person living with HIV takes their medication as prescribed and gets and stays undetectable. It is important to note that U=U only applies to sexual transmission of HIV, and not to perinatal (mother-to-child) and injection drug use transmission.