The biggest medical forum on AIDS kicks off on Sunday to a buzz of excitement about potential strategies for curbing a pandemic that has now claimed 30 million lives in its three-decade history.

A four-day conference in Rome will mull dramatic evidence that drugs designed to treat patients with HIV can be used to shield uninfected people from the AIDS virus.

This could be the best news in 15 years, when antiretroviral drugs started to transform the human immunodeficiency virus (HIV) from a death sentence to a manageable disease.

It opens up dazzling options for rolling back a pandemic for which there is still no cure or vaccine, say some experts.

"Over the past 18 months, there has a been a string of good results from trials, and this has generated a fair bit of optimism," Jean-Francois Delfraissy, director of the French Agency for AIDS Research (ANRS), told AFP.

"Until recently, no one had found a medical way to prevent HIV -- all there was, essentially, was the condom and the message of safe sex."

The conference will gather thousands of specialists, ranging from virologists to pharmacologists and disease trackers.

It is staged once every two years by the International AIDS Society (IAS), which also organises the International AIDS Conference, a bigger event that touches on the pandemic's many social dimensions.

Here are the main causes for all the excitement:

-- a trial conducted among "sero-discordant" heterosexual couples in Africa, meaning couples in which one partner had been tested positive HIV while the other was uninfected.

The risk of HIV infection fell by a whopping 96 percent when the infected partner started early use of daily antiretrovirals.

-- a trial conducted among sero-discordant heterosexual couples in Kenya and Uganda which took a quite different tack. It asked the uninfected partner to take the daily anti-HIV pill.

The risk of infection fell 62 to 73 percent compared with couples where the uninfected partner took a placebo.

"This is a major scientific breakthrough which reconfirms the essential role that antiretroviral medicine has to play in the AIDS response," Michel Sidibe, executive director of UNAIDS, said on Wednesday.

"These studies could help us to reach the tipping point in the HIV epidemic."

Some campaigners say "treatment as prevention" -- treating infected people swiftly so that they do not infect others -- should now spearhead the war on AIDS.

In 2009, more than 33 million people were living with HIV and 2.6 million people became newly infected, according to UNAIDS.

"If you start early treatment for people infected with HIV, you get a 96.3-percent decrease in transmission," Julio Montaner, director of the BC Centre for Excellence in HIV/AIDS in British Columbia, Canada, said in a phone interview.

"Nothing else has been shown that is as efficacious. And the cost benefit is triple, because it decreases morbidity, it decreases mortality and it decreases transmissions. I call it a hat-trick, you can't have it any better."

Now, though, comes the devil of detail, which will fully occupy the Rome conference.

Will the results from a trial -- where volunteers are enthusiastic and encouraged by their partner to follow their pill regimen -- be equally valid when extended to the messiness of real life?

And what about the risk that people become over-confident and forgo use of a condom?

Giving HIV pills to uninfected, as opposed to infected, people raises even more issues.

Antiretrovirals can cause toxic side effects and carry a potentially hefty cost if they have to be taken daily for prevention. The price has fallen to as little as 25 US cents per tablet, but this is still a big discouragement for people who are living on a couple of dollars a day or less.

IAS chief Bertrand Audoin said the prevention trials, combined with proof that male circumcision helps protect men from HIV, showed "we are in the middle of huge scientific breakthroughs that could change the course of the epidemic in coming years."

He cautioned, though: "One of the challenges we have to meet is to align the scientific evidence with policies implemented on the ground."

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