The key determinants of whether HIV transmission is likely to occur from a woman to a man are:

Infectivity

Sex with a partner who has a high level of virus in their blood, either just after infection or as symptoms develop, significantly increases the risk of female–to–male infection.

Sexually transmitted infections

These increase the vulnerability of a man to HIV infection in two ways. They may increase the infectiousness of the female partner (see Sexually transmitted infections above), whilst an ulcer on the penis (such as those caused by syphilis or herpes) increases the vulnerability of the male partner.

Circumcision

Men who are uncircumcised appear to stand a greater chance of HIV infection than men who are circumcised. This may be due to the delicacy of the tissue of the foreskin, and the frequency with which it may be torn during intercourse, particularly when there is a lack of vaginal lubrication. It may also be to do with the fact that vaginal fluid containing HIV may be held under the foreskin after intercourse, increasing the period during which HIV infection may occur.

References

European Study Group on Heterosexual Transmission of HIV: Comparison of female to male and male to female transmission of HIV in 563 stable couples, BMJ 304: 809–813, 1992.

Padian N et al: Female to male transmission of Human Immunodeficiency Virus, JAMA 266(12): 1664 –1667, 1987.

Vogt MW et al: Isolation patterns of the human immunodeficiency virus from cervical and vaginal secretions, AIM 106(3): 380–382, 1987.