before taking HIV+ shots they have to agree with this freely and as i know some of them, they don't care too much. some other studios, the "better ones", strictly reject HIV+ guys from shooting completely.
Then there are some studios (such as Hot Desert Knights, I believe), that will only use HIV+ guys in their scenes. This is troublesome, because someone who is infected with one strain of HIV can conceivably get superinfected with another strain, which would managing the infection much more difficult.