Modern HIV treatment works so well that people with the virus cannot pass it on through sex. Doctors call this U=U: Undetectable equals Untransmittable. Here is the science.

HIV drugs, called antiretroviral therapy, suppress the virus in the blood. When taken daily, the amount of virus can fall so low that standard tests cannot detect it. That is an undetectable viral load.

Three large studies found no confirmed cases of sexual transmission when a partner kept an undetectable viral load. The US National Institutes of Health says such people have “effectively no risk” of passing HIV to a partner through sex.

It usually takes up to six months of treatment to reach undetectable levels. The virus is not cured. It stays in the body, and stopping treatment lets it rebound.

U=U covers sex only. It does not apply to sharing needles, and pregnancy needs separate medical care.

The message matters because stigma still keeps people from testing. In the US, about 71% of newly diagnosed people reach viral suppression within six months. Early treatment protects both the patient and their partners.