Worldwide, the virus affects more than 33 million people, more than two thirds of whom live in sub-Saharan Africa. As a global crisis, HIV/AIDS shows few signs of slowing down.
According to the most recent figures from UNAIDS and the U.S. Centers for Disease Control:
- Approximately 2.7 million people were newly infected in 2007 – more than 7,400 every day. The number of new infections continues to outstrip advances in treatment: For every two people who begin treatment, five are newly infected.
- Globally, women account for half of all HIV infections, and in sub-Saharan Africa, women comprise 60 percent of all infected adults. Young women are especially vulnerable. In southern Africa women aged 15 to 24 are at least three times more likely than their male peers to be infected with HIV.
- More than 2 million people died of AIDS-related illnesses in 2007; 1.5 million of these deaths were in sub-Saharan Africa.
- Between 2004 and 2007, there was a 26 percent increase in the estimated annual HIV/AIDS diagnoses among men who have sex with men (MSM) in the United States. MSM bear the burden of the epidemic in the U.S. and in other parts of the world, such as Europe, Latin America, Australia and New Zealand.
If the numbers don’t tell the whole story, consider that behind each number is a human face. Entire populations are at risk.
At the root of the epidemic is a formidable foe – a virus a million times smaller 
than the period at the end of this sentence yet capable of laying a destructive swath nearly the size of the globe. Although newer drugs have dramatically improved both the quality of life and life expectancy of people with HIV, they aren’t available to everyone needing them, and even for those who do have access to treatment, the drugs don’t always work or they stop working over time. At the cellular level, HIV can play a skillful game of cat and mouse, mutating with frequency in order to evade detection by drug targets. In an attempt to keep HIV from winning in its own game, treatment strategies involve a combination of several antiviral medications that take aim at the virus from multiple directions.
Similarly, prevention efforts, if they are to be successful, will require building a global fortress, with several different defense strategies working in force. Among the approaches being considered are microbicides, substances designed to prevent the sexual transmission of HIV; pre-exposure prophylaxis (PrEP) with an oral antiretroviral (ARV) drug; use of ARVs to prevent mother-to-child transmission; vaccines; and behavior-focused strategies.
How is HIV transmitted?
Most new cases of HIV result from sexual intercourse between couples in which one partner is, knowingly or unknowingly, infected 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. Yet the risks of HIV transmission are the greatest for anal intercourse, particularly among men who have sex with men. There is increasing evidence that heterosexual women in both the developed and developing world also practice receptive anal intercourse. According to some estimates, the risk of HIV associated with unprotected anal sex is 20 times greater than with unprotected vaginal sex.
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. 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 or rectum, but one thin 
layer below, these and other target cells lie in abundance. In the vagina, this layer, called the epithelium, creates a buffer zone that’s a scant 40-cells deep. Merely a single-cell thick, the epithelium of the rectum is even more fragile. 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 what the underlying mechanism is, 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 transmitting HIV through sex 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 or rectal tissue to regional lymph nodes.
