When the amount of HIV in blood (viral load) is less than 400 or less than 50 copies per milliliter of blood (both measurements are considered undetectable and differ only in the kind of test used to determine the amount of virus in the blood). It is generally agreed that having an undetectable viral load, or a significantly lowered viral load, is one way to slow HIV disease progression. Even when a viral load is undetectable, it is still possible to pass HIV through unprotected sex, sharing needles and other risk behaviors.