Most new cases of HIV result from sexual intercourse between couples in which one partner knowingly or unknowingly is ected with HIV. In women, between 70 and 90 percent of all HIV infections are due to heterosexual intercourse. Moreover, women are twice as likely as their male partners to acquire HIV during sex, due in part to biological factors that make them more susceptible.
HIV is particular about the cells it infects. It targets T cells, a kind of immune system cell, and only T cells that have a specific molecule on its surface called a CD4 receptor. The receptor serves as a docking station where HIV parks before invading the cell. HIV then directs the cell’s machinery to incorporate into its genetic blueprint the building blocks for the virus as well. By doing this, HIV ensures that with each cell division the virus multiplies as well.
HIV won’t find many T cells on the surface of the vagina, but one thin layer below, these and other target cells lie in abundance. Just how HIV burrows down below the outer surface to reach its mark is not certain. Researchers propose there may be multiple mechanisms. Perhaps the virus hitches a ride with dendritic cells that straddle the two layers, having conveniently been captured by these cells as an “invader” to be turned over to T cells and other immune cells that would otherwise orchestrate an attack. Alternatively, maybe the virus uses more direct routes through breaks in the tissue caused by local trauma and/or a sexually transmitted infection (STI). A known risk factor for HIV among women, STIs may also enable HIV transmission and infection by signaling in armies of additional target cells as part of the immune response.
No matter the underlying mechanism, an infected cell that migrates to nearby lymph nodes is akin to someone with a highly contagious disease riding public transportation in a large metropolis. In lymph nodes, the virus is exposed to a host of new immune system cells that it can infect, each with the ability to spread the virus elsewhere in the body. Animal models have suggested that initial infection can occur within one hour of exposure, and dissemination of the virus, within 24 hours. Within three weeks of being newly infected, when individuals aren’t likely to have symptoms or know they have HIV, the risk of sexual transmitting HIV is the greatest.
For its many challenges, preventing sexual transmission of HIV is not insurmountable. Compared to other sexually transmitted infections, getting HIV through sexual intercourse is a relatively inefficient process. In addition, the female genital tract is a relatively small anatomical area to protect. Researchers know that for any method to successfully prevent sexual transmission of HIV it must protect the surfaces most at risk; provide a sufficient therapeutic window and prevent migration of infected cells from the local vaginal tissue to regional lymph nodes.