The antiretroviral drugs that revolutionized the care of people with AIDS are on the threshold of a new life as tools to prevent infection in individuals and brake the epidemic in populations as a whole.

Studies are underway testing whether periodic use of the drugs, either as pills or as vaginal or rectal gels, can prevent transmission of HIV in high-risk sexual encounters. At the same time, it's becoming clear that the incidence of HIV infection declines over time in places where most infected people know their status and are on treatment -- and thus are less likely to pass the virus to others.

Description of these effects at a big AIDS conference here is likely to spur a further swing of the treatment pendulum toward early and widespread treatment of HIV infection.

"Arguably the greatest progress in the AIDS epidemic has been in the development of highly effective drugs," said John W. Mellors, an AIDS researcher at the University of Pittsburgh and chairman of the 17th Retrovirus Conference. "This is now being applied not only to help infected individuals but as a public health approach to the whole epidemic."

Since the advent of "combination antiretroviral therapy" (ART) in 1996, patients and physicians have debated how aggressively the drugs should be used.

Recognition of ART's dramatic life-extending effect was followed by the appearance of unexpected side effects, including increased risk of diabetes and heart disease and changes in many patients' appearance. As a consequence, researchers over the last 15 years have conducted dozens of studies seeking to learn how long an infected person could safely put off starting the drugs, or whether an infected person could stop taking them periodically without harm.

It's now clear that interrupting treatment is not a good idea and that starting it early in the course of infection may have real benefits, even though it means a lifetime of daily pill-taking. The new studies discussed here are likely to only increase the use of the drugs, which now number more than 30.

Antiretroviral drugs are already being given to babies born to infected mothers immediately after birth and during breast-feeding to greatly reduce the chance of infection.

"We know prevention works in babies. It shouldn't be any different in adults," Mellors said.

Several studies are underway testing "pre-exposure prophylaxis" with the drugs in people at high risk for acquiring HIV, including commercial sex workers. Some results may be available late this year.

There are already indirect hints that AIDS medicines can be prevention tools just like condoms and abstinence. Specifically, when an infected person is on a successful ART regimen, the amount of the virus in the bloodstream falls to such a low level that the chance of infecting someone else is almost nil.

In a study presented Friday, Deborah Donnell of the Fred Hutchinson Cancer Research Center in Seattle described this effect in "discordant couples" -- a couple in which only one partner is infected -- in seven African countries. When a person's HIV infection got to the stage at which ART was started, the chance that the partner would become infected fell by 92 percent.

This suggests that, at least in discordant couples, ART should be started much earlier than guidelines in African countries recommend. A similar argument could be made for all HIV-infected people in those high-prevalence countries.

Such a strategy would require a huge step-up in treatment.

The World Health Organization recommends that infected people start ART when their CD4-cell count -- a gauge of the immune system's health -- falls below 350. Now, only 30 percent of HIV-infected people in that range are getting the drugs.

Antiretroviral drugs are also breathing life into the quest for a vaginal "microbicide" that a woman could apply, in private and without her partner's knowledge, to prevent HIV infection. Numerous substances have been tried and none has worked. Researchers here described experiments in which rhesus monkeys were protected from infection with a microbicide containing maraviroc, an HIV "entry inhibitor." Taken in pill form, the drug also is concentrated in rectal tissues, suggesting it may be useful to prevent infection through anal intercourse.

One of the researchers, John Moore of Weil Cornell Medical College in New York, said his team is now working on a maraviroc-containing vaginal ring, which would release the protective drug over a period of weeks.

A potentially dramatic effect of widespread ART use was described by Julio Montaner, director of the British Columbia Center for Excellence in HIV/AIDS, which oversees treatment of virtually all HIV-infected people in that Canadian province.

In the years immediately after 1996, when the province made a big push to get HIV-infected people on antiretroviral drugs, the number of new infections fell by one-half -- probably because there were fewer people with high "viral loads" of HIV available to infect others. There was a second decline after 2004, when it became clear that starting ART earlier and not stopping it was the best strategy. Since 2007, there has been a 50 percent drop in new infections among injection drug users, who are the group with by far the highest risk of acquiring the virus.

This trend is strong support for the new paradigm of aggressively seeking out infected people and offering them ART. It may be expensive upfront but, Montaner said, "the effect of preventing one infection is so great that it becomes potentially cost-averting."