REBECCA, an attractive woman in her late twenties, stares hard at the ground, tears in her eyes. "Shaun told me about his affair four months ago. He called it his 'mistake', but then he told me he'd caught an STI from her, which turned out to be chlamydia. I was shocked and angry that this could happen to people like us," she says.

Rebecca and Shaun had been married for two years when he dropped that bombshell. The couple are becoming increasingly typical of the kind of people finding out they've been infected with conditions like chlamydia, gonorrhoea, syphilis or herpes. Rebecca's husband may have only been alerted to his condition by chance, as many people infected with chlamydia have no symptoms.

"Shaun admitted that he'd eventually seen his GP after several weeks suffering from a burning sensation 'down there'," says Rebecca. "I was horrified that he'd brought something so dirty home with him."

Rebecca herself was tested a couple of weeks after first experiencing pelvic pain, which at first she didn't put down to an STI. She tested positive.

Shaun started seeing another woman after meeting her at work. Others may throw all caution to the wind when starting a new relationship, while a core group of men and women indulge in alcohol-fuelled bed swapping, especially on holiday. Such carefree behaviour is often fuelled by binge drinking or drugs, yet when the effects wear off, they're left with a nasty legacy.

"Chlamydia is now the most common, curable sexually transmitted infection in people under 25 in England and Wales, with around one in ten young people being infected," says Lynn Wilson, leader of the County Durham Chlamydia Screening Programme. "The rate of chlamydia infection has almost doubled in County Durham over the last five years, mirroring the national increase."

Conditions like chlamydia or gonorrhoea are a ticking time bomb when left undetected. The problem is that as long as people ignore the risks they're taking with their health, the situation will only get worse.

According to the latest figures from the Health Protection Agency, cases of chlamydia rose by nine per cent to 90,000 in 2005.

The good news is that the condition can be successfully treated with a course of antibiotics. The Government is now promising a screening programme for the condition by 2007, and home testing kits are starting to appear on the high street.

Lynn says: "It is important that people find out as soon as possible if they have chlamydia as it may have serious long-term consequences, including pelvic inflammatory disease, ectopic pregnancy and infertility in men and women."

Worryingly, cases of syphilis, which health professionals thought were steadily declining, leapt by 28 per cent last year. The disease, which can cause heart problems and dementia if left untreated, once held Victorian Britain in its grip. It now threatens to become a contemporary plague. It seems the iconic AIDS campaigns of the early eighties, which scared many people into total abstinence, have now been forgotten. Professor Mark Bellis, director for the Centre of Public Health, in Liverpool, says that more should be done to target the "promiscuous ten per cent" of the population. But he admits that people who have multiple partners and use prostitutes are often the last to listen to a public health message.

The Department of Health has tried to counter this behaviour with messages like 'Don't Play The Sex Lottery: Use A Condom', and the website www.playingsafely.co.uk, but campaigns like 'Roger More' for Durex Condoms have been withdrawn after vociferous complaints.

Meanwhile, the message going out from the Government and health campaigners is that young people in particular should come forward and be tested, before it's too late