The trafficking of young girls who are forced to work as prostitutes has been identified as a key factor in the steep rise in Aids in India.
By Matthew Grant
BBC correspondent in Calcutta
Looking for business - Calcutta prostitutes
The country already has about 5.1m people who are HIV-positive - the second highest number in the world after South Africa. Some estimates predict this could rise to 20m in just six years.
In big red light districts, such as Sonagachi in Calcutta, where at least 10,000 prostitutes make a living, some men continue to insist on sex without condoms.
The trafficked girls are forced to oblige. Many come from rural villages and do not know what Aids is before they are sold to pimps.
And as they are moved around the country they can unwittingly spread the disease.
In eastern India, Calcutta has emerged as a hub for the trafficking of girls, who also arrive from Nepal, Bangladesh and Burma.
From Calcutta they are often sold again to brothels in Mumbai (Bombay).
Some will go on to the Middle East, Africa and Europe.
Aklina Khatoom is a small, pretty 15-year-old girl from a village outside Calcutta.
Aklina (left) with one of the women who helped her escape prostitution
In a soft voice, she recalls how a year ago a woman drugged her, kidnapped her and sold her to a madam in Mumbai.
"I was then told that I would have to become a prostitute and I said that there was no way I would do that," she says.
"But I was beaten so much, I was slapped, my whole body was covered in bruises, then they used hot iron rods to hit me - eventually I had to agree to it."
Aklina could not escape, as she was guarded by the sister of the woman who sold her.
"My day began at six in the morning and I had about 12 to 14 customers on a daily basis and my day ended at 3am."
Her luck changed when a customer let her call her parents from his phone. Finally she was rescued, but her ordeal had not ended - there was the threat of Aids infection.
According to one estimate, 70% of prostitutes in Mumbai are infected with the virus.
"I didn't know what Aids was before I went to Mumbai," Aklina says.
"Once I got there and I got to the brothel I became aware of it and I was so afraid it would be something I would catch.
"After I was rescued and I came back home and I told my mother everything. She had me tested and fortunately I was negative."
Stigma of sickness
Aklina is one of more than 300 girls rescued from traffickers by Swapan Mukherjee and his Calcutta-based organisation, the Centre for Communication and Development.
Poverty and illiteracy often force families to part with their daughters for false promises of work or marriage, Mr Mukherjee says.
In return the families receive as little as $10.
Even after they are rescued, girls like Aklina face the stigma of having been forced to become prostitutes.
But those who become infected with HIV are shunned completely.
When Swapan Mukherjee let journalists interview some girls he had rescued their openness had disastrous results.
"That time it was 34 girls that we'd rescued. And one girl who was illiterate just said, 'yes I am HIV positive'. When this news appeared in the newspaper, the villagers isolated that family.
"Finally the whole family was facing terrible isolation and so to save her family she had to go back to a brothel in Mumbai."
The girl is now dying. She recently called Mr Mukherjee, but he says it is too late to save her.
There may yet be time to help the girls still working in India's red light districts, but it will be a difficult task.
Some 10,000 prostitutes work in Calcutta's red light district
In Sonagachi in Calcutta, the efforts of the local sex workers' union, the Durbar Mahila Samanwaya Committee, to promote the use of condoms have won worldwide acclaim.
The group's secretary, Swapna Gayen, says the women will not have sex with men who refuse to use condoms.
But a visit to one of Durbar's own clinics in the heart of Sonagachi shows this is not quite the case.
A prostitute called Mituraj, who says she is 20 but looks younger, is there to get treatment for syphilis. She has never taken an HIV test.
"I am afraid of Aids," she says.
"I do know what is. But sometimes what happens is that we don't get any customers if we use condoms.
"It has happened to me that for three or four days I haven't had any customers, so then I have had to accept sex without a condom."
The bad news for Aids campaigners is that the disease is no longer confined to prostitutes and red light districts.
It is spreading rapidly throughout India - and so long as the trafficking of young girls continues, there can be little hope this huge number of infected people will do anything else but go on rising.