The latest world estimates for the spread of HIV and Aids show there are 10,000 Britons infected without knowing it who are consequently oblivious to the risks they pose to their sexual partners.
Ignorance is an even greater problem overseas. More than 95 per cent of the 33.5 million people estimated to be carrying the virus worldwide are unaware that they are infected, according to the Joint United Nations Programme on HIV and Aids.
In Britain, where HIV tests are readily available, a total of 30,000 adults are known to be living with the virus.
The Department of Health and the UN Aids programme both emphasised that the continuing spread of HIV is being encouraged by a growing complacency about the disease, largely resulting from the development of anti-viral drugs.
Dr Angus Nichol, the head of the HIV division at the government's Public Health Laboratory Service in north London, said new drugs had cut death rates sharply but had done nothing to prevent new infections occurring.
"We continue to see between 2,500 and 3,000 new diagnoses of HIV in the UK each year, and there is a real danger of complacency," Dr Nichol said. Dr Peter Piot, executive director of the UN Aids programme, said the latest global estimates of HIV undermined the belief that the epidemic was over. "The epidemic is far from over. The crisis is growing," he said.
The World Health Organisation (WHO) estimates that, since the Aids epidemic began, 50 million people have been infected with HIV. Of these, 33.6 million are still alive.
Last December, the WHO estimated that 5.8 million men, women and children had been newly infected in 1998, but this figure has since been revised down because of what were seen as over-estimates of infection rates in India and parts of South East Asia, Dr Piot said.
"There is no room for complacency in any discussion of this epidemic. The threat of HIV has not diminished in any country. We have even seen evidence from North America and Western Europe suggesting that availability of life-prolonging therapies may be contributing to an erosion of safer sexual behaviour," he said.
The latest WHO report, Aids Epidemic Update, reveals that the fastest growing epidemic is now occurring in Eastern Europe and the countries of the former Soviet Union.
The proportion of the public infected with HIV in the former Soviet Union doubled between 1997 and 1999. In Russia, nearly half of all reported cases of HIV infection since the beginning of the epidemic were recorded in the first nine months of this year alone.
An explosive growth in the number of injecting drug users in Russia is fuelling the steep rise in new HIV infections, which have been identified as far east as the Siberian city of Irkutsk, where nearly 1,300 people have been infected in 1999.
Dr Piot said sub-Saharan Africa still carried the greatest brunt of the Aids epidemic, with 23 million people infected.
For the first time, the WHO has reported that women in Africa are significantly more likely than men to be HIV positive. For every 10 African men infected with HIV, there are 12 women infected, it says.
Scientists cannot fully explain this, but say it may be due to a combination of factors such as the virus being more easily transmitted from men to women and the high incidence of young girls in Africa being coerced into having sex with older men.
"Ten years ago it was hard to make people listen, when we were saying that Aids wasn't just a man's disease. Today we can see the evidence of the terrible burden women are now carrying in Africa's epidemic," Dr Piot said.Reuse content