Question

I had a sex with my girl friend in Shanghai last week and now I suspected her is a HIV positive patient. I just wondering if the probability of transmission from woman to man is significantly lower than vice versa? I heard that man's genital organ is covered with regular skin except urethral opening which is made of mucuous membrane, in consequense, it is very hard to catch AIDS for man even during unprotected sex. Can you explain more about this in biological aspect?

 
Answer

 
You are correct. According to results from the Northern California Partner's Study, published last year, (Padian, NS, Shiboski, SC, Glass, SO, Vittinghoff, E. Heterosexual transmission of human immunodeficiency virus (HIV) in northern California: Results from a ten-year study. American Journal of Epidemiology. 1997; 146(4): 350-57.) Male-to-female transmission was approximately eight-times more efficient than female-to-male transmission and male-to-female per contact infectivity was estimated to be 0.0009 (95% C 0.0005-0.001). This is due to the fact that the lining of the vagina is a mucous membrane and hence more permeable than the outside of the male penis. However, if any irritation or lesions are present, say a herpes sore or a yeast infection, the risk of transmission is greatly increased.

As the partner's study noted:

History of sexually transmitted diseases was most strongly associated with transmission Infectivity for HIV through heterosexual transmission is low, and STDs may be the most important cofactor for transmission.

Another factor is that women have more surface area through which infection can occur. Lack of lubrication during intercourse, youth, changes in cervix during menstruation, and a general lack of access to regular GYN care for many women are also risk factors placing women at greater risk than men.

The STD risk factor holds for men as well. While female to male transmission is rare, I have had to give positive results to a guy who's female partner apparently gave him herpes and then HIV. This all came as a rude awakening for both of them. Therefore, it's important to use condoms until both you and your monogamous partner have tested negative for HIV and STD's.