CALCUTTA, India, July 11 (Reuters) - Radha Singh was just 15 when she was forced to become a prostitute after her parents died.

Today, the 40-year-old mother of two says nothing fazes her any more after 25 years in Calcutta's red light area of Sonagachi.

Nothing - except the spectre of AIDS.

"I am so very fearful of AIDS. This is one disease you just can't recover from. That is really scary," said Singh, dressed in a white sari, her red and white bangles jangling as she spoke. Just some distance away, a group of sex workers - some only in their teens - sit on windows smiling at potential customers.

Sonagachi, a bustling red light district with some 15,000 sex workers, is one of the battlegrounds in the war against the HIV epidemic in India where 5.1 million people are infected with the virus - the second largest number after South Africa.

According to the U.S. Central Intelligence Agency, that figure could soar to over 20 million by 2010 as the vast majority of HIV-positive Indians are unaware they are infected and many do not have access to anti-retroviral therapy.

The Indian government has a nationwide programme to halt the spread of the disease, but over the years HIV/AIDS has moved beyond traditionally high-risk groups such as prostitutes, drug users and homosexuals.

Experts say the most alarming trend is the spread of the disease to the countryside due to the country's large migrant population, with rural India accounting for a high 59 percent of infections in 2003 compared with 41 percent in urban areas.

While there has been a huge increase in the number of infections due to injectible drug use, around 80 percent of HIV infections in India are still through the sexual route.

But the project director of India's state-run National AIDS Control Organisation says the situation is not alarming.

"There is no galloping HIV epidemic across India and there is no evidence of an upsurge in HIV prevalence in any state in the country," said Meenakshi Datta Ghosh.

"It's unfair to project it (India) as a ticking time bomb."

Unlike South Africa - where 5.3 million people live with HIV making up over 11 percent of the population - less than one percent of India's huge billion-plus population is HIV-positive.

INVOLVED IN THE FIGHT

One of the success stories in the fight against the spread of HIV among sex workers - the most high-risk group - is in Calcutta where infection rates have been stable since 2000.

Compared to the HIV infection rate among sex workers of over 40 percent in southern Tamil Nadu state, just eight percent of Calcutta's estimated 25,000 prostitutes are HIV-positive.

A key focus of the city's anti-AIDS campaign has been to get prostitutes to band together to ensure clients use condoms and, if they don't agree, refuse them sex.

"We fear AIDS and so should our customers," 33-year-old Rekha Lodh from Sonagachi said.

One of the main factors for the city's success is the involvement of sex workers themselves in HIV-intervention programmes, says Smarajit Jana, chief adviser to the Durbar Mahila Samanwaya Committee (Indomitable Womens' United Committee), India's biggest forum for sex workers.

"They feel AIDS is their issue as it affects their lives."

But at the other end in Bombay, India's business hub on the western coast, the problem is still severe as more than half the city's 20,000 prostitutes in the city are infected.

"In Mumbai (Bombay), it seems a case of sustained prevention failure," Elizabeth Pisani of the Family Health International told Reuters from Bangkok, which is hosting the 15th World AIDS conference from July 11-16.

Many migrants in cities like Bombay, which has a huge floating population that hinders an effective AIDS campaign, are infecting their wives in rural areas after sex with prostitutes.

"Many times, the husband does not know he is carrying the virus because of a long asymptomatic period of about eight years. So many young newly-married girls are infected," Dr Alka Deshpande, an expert in AIDS medicine in Bombay, told Reuters.

IGNORANCE, STIGMA

Experts say India cannot afford to be smug about its low infection rate as ignorance about AIDS is common and a huge stigma is attached to those who are HIV-positive.

In the grimy town of Malda, some 320 km (200 miles) south of Calcutta, rickshaw puller Ashok Mandal has never heard of AIDS.

"AIDS? What is that?" Mandal said.

In southern Tamil Nadu state, a woman killed herself last year after she and her son were ostracised by neighbours following the death of her husband due to AIDS.

"Africa is a much more open society, so they have woken up to it (AIDS). In India, there is a social stigma about sex," said J. Vimalanathan, executive director of New Entity for Social Action, which networks 42 voluntary organisations in southern India.

For some, the horror of AIDS has already hit home.

"I don't want to suffer more. I feel very weak, the pain in my (infected) ear never stops and I cannot breathe properly," a gaunt Kamala Basappa, a 33-year-old former prostitute in Bombay, detected as HIV positive six years ago, told Reuters. (Additional reporting by Jayashree Lengade in BOMBAY and Narayanan Madhavan in BANGALORE).