Up to 30 per cent of Americans newly infected with HIV now carry a form resistant to at least one of the 13 drug combinations that have been revolutionising HIV therapy, a US conference was told last week. And the numbers of people being infected are back on the increase due to a unholy combination of complacency and party drugs.

New research also confirms hitherto anecdotal evidence that some HIV-positive people are shunning safe-sex practices, and passing on drug-resistant strains of the virus. Experts believe the high incidence of resistant virus in circulation in the US is largely due to the bug's phenomenal ability to change its shape when threatened by the drug-boosted immune system - virologists have identified 125 distinct mutations since 1992. But mutation rates are also boosted when patients don't adhere rigidly to the drugs' daily dosing regimen, or the drug combination chosen does not significantly reduce virus levels, allowing particles to escape and mutate.

'Resistant virus is prevalent in the population and is being transmitted,' John Mellors, a researcher at the University of Pittsburgh, told the sixth Conference on Retroviruses and Opportunistic Infections in Chicago. Though such transmission is 'not common', he added, 'it is likely to spread, and the frequency will go up.' These findings come amid reports of rising levels of unsafe sex among men in the US.

According to a report last month from the Centres for Disease and Prevention [CDC] in Atlanta the number of gay men in San Francisco who reported having unprotected sex increased from 30 per cent in 1994 to 39 per cent in 1997. And a study of HIV-positive men on combination drug therapy in one city, published in the journal Aids last autumn, reports that almost one in five were less likely to have safe sex.

Doctors are concerned that a growing number of HIV patients are becoming careless about safe-sex and the risks of passing on the virus because drug cocktails have lulled them into a false sense of security. The result is that the rate of new HIV infections in the US is steadily rising even though those of full-blown Aids and deaths from Aids have fallen.

Part of the problem, doctors say, is that HIV patients know too much about their disease and treatment. Immersed in information about viral load and CD4 counts, they are aware that the drugs can make their blood seem clear of infection.

'It appears unlikely that highly active antiretroviral therapy (HAART) is significantly reducing the rate of new HIV infection in the US at the present time,' Dr Jonathan Kaplan of the CDC told the conference 'It has become apparent in the past year that many persons receiving HAART may perceive they are of less risk to others.' 'Today's young folks did not grow up in the trenches of watching people die,' said Charles Dickey, a psychotherapist in Atlanta whose clientele is mostly gay. 'They don't see Kaposi's sarcoma. They don't see people wasting away. Back in the old days, when people were dropping like flies, there was real fear. Now people think HIV is no big deal, that it's like diabetes. You take some pills and it's no problem.' Aids caseworkers, doctors and therapists are also concerned that the use of new inhibition-relaxing drugs like ecstasy, gamma hydroxybutyrate (GHB), ketamine and crystal methamphetamine, is encouraging young gay men to indulge in high-risk sexual behaviour.

In Britain, rates of newly-acquired HIV are also starting to increase again. According to Paul Ward, spokesman for the Aids research charity, the Terence Higgins Trusts, two main groups - gay men and African people - account for most of the new cases with the latter accounting for the lion's share of the increase. 'Seventy to 80 per cent of new cases among African people are thought to have been acquired in Africa,' he says.

In the US, by contrast, the disease is evolving from being largely an illness of white homosexuals to one of poor blacks who catch it through drug abuse and heterosexual encounters. Nationally, more than half of all HIV infections are now among black people.

Whether British gay men are succumbing to the same safe-sex complacency as their American peers is less clear, says Ward. While there's anecdotal evidence of a rise, THT's annual survey of 5,000 gay men has found no change in either 1998 or 1997.'But now there's hard research from America and from Australia too, we have to expect to see increasing levels of unprotected sex here. If everyone was having safe sex, rates should be falling.'