The numbers in India are frightening: In a country of more than 1 billion people, some 5.7 million are infected with HIV/AIDS. That makes India home to more victims of the disease than any other country in the world.

Set against those statistics is an army of people trying to fight the virus. Backing some of them are hundreds of millions of dollars from the world's deepest philanthropical pockets — The Bill and Melinda Gates Foundation.

The foundation's AIDS prevention effort in India, known as "Ahavan" — a Sanskrit word meaning "call to action" — has a $200 million five-year grant to operate an HIV prevention program on a scale never done before. But in a country as vast as India, it could do with more.

The AIDS fight here could easily cost $1.5 billion a year, said Ashok Alexander, the foundation's India director, citing recent U.S. government figures.

The challenge is complex. Often the sex trade — a main transmission route for AIDS — is nearly invisible, with prostitutes working out of truck stops, or even in small village homes. Some sex workers are highly mobile, moving from city to city.

One of India's hardest-hit areas, its remote northeast, has an entirely different problem. There, most HIV transmissions come from needles shared by thousands of heroin addicts, a problem fueled by the region's proximity to the poppy fields of the Golden Triangle.

Logistics can also be nightmarish in rural areas that often lack basic infrastructure.

All these problems are confounded by the stigma attached to AIDS in a very conservative country.

Ahavan's strategy has been to adopt a business-style structure. The product is prevention, and the foundation formed a pyramid structure to get it to consumers. It contracted 15 other organizations that, in turn, work with about 150 grass-roots groups. They employ some 5,000 prostitutes, many of them HIV-positive, to get the message out.

"I talk to women about condoms and how they must insist even their regular clients wear condoms," said Vijaymala, a fruit seller who supplements her income by working as a prostitute without her family's knowledge.

She has a third job working as a "peer counselor" at Saathi, a tiny Gates-funded clinic nestled among the shantytown brothels in Turbhe, an industrial area on Bombay's outskirts.

Employing prostitutes means feedback comes quickly when there are problems.

"In talking with sex workers, our team found that the women felt the condoms available in the market did not suit them," said Sanjeev Gaikwad of Family Health International, which runs the Saathi clinic.

So they went to a condom manufacturer to produce the stronger, better-lubricated ones they now distribute.

Alexander said that after three years of work, it's too soon to evaluate how successful Ahavan has been, but he is positive. "We started off with a five-year grant, but I think we will be here as long as it takes," he said.

The Indian government — notoriously testy about accepting help from outsiders and often at odds with AIDS activist groups, accusing them in the past of exaggerating the numbers — has nothing but praise for the Gates Foundation.

"They are playing a very critical role. The foundation is one of our major partners in helping us reduce the transmission of the disease," said Sujata Rao, director general of the government's National AIDS Control Organization.