In ''Langerhans Cells Lap up HIV-1'' (Nature Medicine, vol. 13, no. 3, March 2007), Olivier Schwartz of the Pasteur Institute identifies a protein that acts as a natural barrier to HIV infection. The protein is called langerin, because it is produced by Langerhans cells, which form a network in the skin and mucosa (the membrane lining the vagina) and were previously thought to promote the spread of HIV. Instead, the Langerhans cells, which line the human genital tract, contain a protein that eats viruses. Langerin scavenges for viruses in the surrounding environment and thereby helps prevent infection. Langerhans cells do not become infected by HIV-1 because they have langerin on their surfaces. It appears that HIV infection occurs when levels of invading HIV are high or if langerin activity is especially weak. In either of these instances Langerhans cells can become overwhelmed by the virus and infected.

HIV is classified as a communicable sexually transmitted disease in the United States. Some physicians prescribe the drugs used to treat established infections as ''morning-after'' pills in an attempt to prevent the transmission of the virus after risky sexual encounters. As of 2007, there was no scientific consensus on the validity of this approach, and the medications were not licensed for this use. Because some forms of HIV are halted by prompt use of the drugs, some doctors believe it is a worthwhile approach. For example, Tony Hosey indicates in ''Lowering the Risk of HIV after Sex or Other Exposure'' (Positively Aware, vol. 16, no. 2, March-April 2005) that an antiretroviral drug regimen given to hospital personnel following an accidental needle stick from an HIV-infected patient seems to reduce the risk of transmission by about 80%.

This ''off-label'' use of potent PIs in an attempt to prevent the spread of HIV is controversial. All drugs used in the treatment of HIV have side effects, some of which may be potentially life threatening. Furthermore, some researchers fear that if people believe morning-after treatment will prevent HIV infection, they may stop taking precautions, such as using condoms, to prevent exposure to HIV. Others feel that the treatment is not appropriate as a preventive measure for people exposed to ongoing risk, such as relationships where only one partner is infected, because the drugs are too toxic. Other methods, such as the continued use of condoms, would be much safer.

Finally, postexposure treatment is expensive. The costs of two or three drugs taken for a month, plus laboratory tests and visits to the doctor, may run as high as $1,000. Of course, this is a fraction of the cost for lifetime treatment of HIV infection and certainly money well spent if it prevents a person from acquiring the virus.

Some researchers say the morning-after treatment could save lives. The drugs themselves will save some lives, and the offer of treatment will bring people who are at high risk for acquiring HIV into environments where they can get counseling and care. In San Francisco, California, postexposure treatment is offered to victims of rape as a matter of course. Some doctors feel that if the treatment does not work to prevent the disease, it may work to at least treat it as early as possible. Though there is disagreement about the effectiveness and wisdom of widespread use of postexposure treatment, nearly all researchers and healthcare providers agree that for sexually active people the best prevention is the use of condoms.

In ''HIV Postexposure Prophylaxis in the Emergency Department: The Morning after Is Today'' (Annals of Emergency Medicine, vol. 42, no.5, November 2003), a survey of emergency room physicians, Joshua D. Bamberger of the San Francisco Department of Public Health finds that most physicians believe that offering post-HIV exposure prevention is both feasible and among their responsibilities. Because physicians' confidence about their abilities to assess the need for postexposure prevention varies with exposure type, Bamberger recommends establishing postexposure prevention protocols and providing education to improve their knowledge of prudent postexposure prevention practices.

In recent years there have been many efforts to develop topical microbicides--preparations to prevent HIV infection. Lawrence K. Altman reports in ''Tests of Drug to Block H.I.V. Infection Are Halted over Safety'' (New York Times, February 1, 2007) that in 2000 a clinical trial revealed that nonoxynol-9, a topical preparation used to prevent pregnancy that initially appeared to help protect against HIV, was unsafe when used to prevent HIV infection. Ironically, participants in the clinical trial developed higher rates of HIV infection, most likely because the chemical caused an irritation that compromised the mucosa, allowing the virus easier entry. In response to these findings, the clinical trial was halted.

Altman indicates that studies were under way in 2007 to test other topical microbicides because until there is an effective AIDS vaccine, preventing transmission of HIV is an urgent public health priority, especially for women in developing countries, where traditions and cultural norms do not support the use condoms.

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