Contact blood-blood should be. Purely theoretically, if you and a friend decide to get high, and both have gums or lips bleeding, then one can pass the baton to the other and safely rot. But it should be just an enchanting combination of circumstances.
It's easy, when smoking hashish, you can fuck in the ass with your friend who has AIDS. There are also other options, for example, first pounding in the butt, and then smoking hashish, so, as the saying goes: "God himself commanded."