It is the uninvited guest that slips in silently and begins to destroy from within. Now, having all but ruined one continent, HIV/Aids looms over another. While the ravaging of Africa by the human immunodeficiency virus, that leads to Aids, is a familiar story, a fresh and worrying chapter is being written in Asia. Of the countries here, India, where these pictures were taken, is experiencing a severe and little-reported Aids epidemic.

As of 2003, there were an estimated 5.1 million HIV-positive Indians. To put that in perspective, the figure for South Africa, the country with the most HIV-positive people, is 5.3 million (out of a population of 44 million). Given India’s population — 1.08 billion and projected to overtake China’s this century — even a low prevalence of HIV is catastrophic. And just as African economies have suffered through a sickening of the workforce, the virus imperils India’s status as an emerging economic powerhouse. It is no coincidence that the 2005 UNAIDS/WHO report, regarded as the most accurate annual compendium of HIV-related statistics and trends, was launched last month in the Indian capital, Delhi.

India’s Aids crisis is being fuelled by the sex and drugs trades, and the crossover between them (drug users often sell sex to fund their habit). In Bombay (now known as Mumbai), 52 per cent of sex workers are thought to be HIV-positive; 42 per cent of sex workers nationally believe they can tell whether a client is HIV-positive from his appearance.

This is the danger of HIV — it slowly and invisibly sucks the life out of a person’s immune system. It can take as long as ten years for HIV to progress to Aids (acquired immune deficiency syndrome), the terrifying stage at which the number of immune cells fall below a threshold level. Victims, their immune systems shredded, can succumb to many Aids-related illnesses, including pneumonia, wasting syndrome and cancers. By the time an infected person knows that something is wrong, it is too late, both for him and the others that he has infected. There is no cure and no vaccine that protects against infection.

India, the Land of the Untouchables, is a perfect place for the virus to propagate. The ability to stigmatise comes naturally to a society still wedded to the caste system; accordingly, HIV-infected people now constitute another layer in India’s underclass. Such communities — uneducated, poor and shunned — present a simmering reservoir of infection.

Sexual taboos reign — it is only recently that censors relaxed the Bollywood ban on onscreen kissing. Premarital sex is frowned upon — one of the fastest growing HIV-positive groups is young women, infected by husbands who used prostitutes before marriage.

In another depressing echo of Africa, Indian truck drivers who criss-cross the country delivering goods and buying sex at dhabas (truck stops), are helping to spread the virus. In Assam, a quarter of truckers are HIV-positive.

States in the south and west of India show particularly worrying infection rates; the virus is also spreading into rural areas. In Andhra Pradesh and Tamil Nadu, for example, 1 per cent of pregnant women are HIV-positive.

There are glimpses of hope. In the red-light district of Calcutta (now known as Kolkata), 85 per cent of prostitutes now use condoms; HIV among sex workers has fallen from 11 per cent in 2001 to 4 per cent in 2004. Still, overall, the picture is not promising - an estimated 30 per cent of those in the sex trade remain unaware that condoms stop the spread of HIV. The Gates Foundation has pledged $200m to an Aids prevention programme, which will promote behaviour change and condom use among truckers and sex workers.

The global Aids epidemic is entering its 25th year. While affluent countries have the disease under control through nationwide testing, medication and public education programmes, the war is being lost elsewhere. The UNAIDS/WHO report estimates that, currently, 40.3m people worldwide are infected with HIV, the highest ever number. Frighteningly, only one in ten knows that they are infected. This year alone, 3.1 million worldwide died of Aids-related illnesses, of which 570,000 were children. Africa is still bearing the brunt — two-thirds of new infections occur here.

Of the estimated 600,000 Indians who need antiretroviral medicines which boost immunity, only 100,000 are getting it. The global hunt for a vaccine continues, but the constantly mutating virus — originally a simian virus — is a notoriously difficult immunological target. This year GlaxoSmithKline and the International Aids Vaccine Initiative announced a collaborative effort to find a vaccine against African HIV strain, but success will be years away. Urgent prevention programmes, therefore, are seen as the most effective way to contain India’s new scourge, and stop the soon-to-be most populous country from becoming the most Aids-riddled nation on Earth.

World Vision, the international aid and development agency, has worked in India for more than 40 years. To find out more or to donate to its work in India or around the world, go to www.worldvision.org.uk or call 0800 501010