The answer is yes. If the person with HIV is on HIV medications and has been able to maintain an undetectable HIV viral load in their blood for longer than 6 months, then they cannot transmit HIV.  This has been well documented in large studies of people with HIV who have HIV negative partners. HIV medications work to reduce the HIV viral load and in most cases the medications work well to bring the HIV to an undetectable level in the blood. Sometimes people with HIV choose not to take the medications and sometimes the medications do not work to bring about an undetectable status. If the person with HIV has a detectable HIV viral load in their blood then they can transmit HIV through their blood and sexual fluids. In this case, avoiding contact with the vaginal fluids by using condoms for penetrative sex will prevent HIV transmission. For more information about undetectable equals untransmittable, please see http://sfaf.org/hiv-info/basics/what-is-undetectable.html