Factors that raise the risk are recent
HIV infection (within 3 months), advanced
HIV disease, STDS (especially genital HSV-2) in either partner, and if the male partner is uncircumcised. Things that reduce it are for the infected person to be taking anti-HIV drugs, to have a relatively healthy immune system, and to have a low HIV viral load (i.e., not much virus in the blood) -- and, of course, consistent condom use.