Biological risk factors associated with the sexual transmission route
Viral load refers to the amount of the virus in the blood. People who have high amounts of virus in their blood are more likely to transmit the infection to others. As we saw in our earlier discussions, HIV-infected individuals have especially high amounts of virus in their blood soon after infection during the window period, and again at the terminal stages of the natural disease progression. It also happens when there is re-infection with a new virus. High viral loads increase the risk of transmission for all routes.
In penile–vaginal sex, women are more susceptible to getting HIV than men because of the structure of the female genital tract: greater exposed area, more mucous membrane; the vagina is a receptive organ. In penile-anal sex, the receptive partner is more susceptible because of the fragile nature of the lining of rectum.
Young women (adolescents) are particularly susceptible because the genital tract is immature and the skin is delicate. Men are also at increased risk for acquiring the infection from an infected young girl because of the risk of bleeding from tears in vaginal mucosa or skin.
Sex during menstruation
The woman is more vulnerable as a result of the change in the lining of the uterus, and the man is also more vulnerable because of exposure to blood in addition to vaginal fluids.
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Uncircumcised men are more easily infected. Research has shown biological explanations for a link between HIV infection and lack of circumcision. The inside of the foreskin can absorb HIV very efficiently, mainly because it contains HIV "target cells" in much greater quantities than other genital tissue. Also, the internal foreskin