When the Mood Strikes; Teens and young adults are now the group most threatened by sexually transmitted infections.

Byline: Emily Flynn and Malcolm Beith (With Karin Bennett In New York, Joe Cochrane In Bangkok, Khieu Kola In Phnom Phen, Marites Vitug In Manila, Sarah Schafer In Beijing, Sudip Mazumdar In Delhi, Mac Margolis In Rio And Marina Suponina-Calland In London)

Hanging out with their friends in East London, Aysha and Sara can seem wise beyond their teenage years. They speak passionately, for instance, about the plight of children in Iraq and Sudan. But when the topic turns to sexually transmitted infections, they're as naive as they come. Aysha, 15, insists that there's a cure for AIDS. Neither girl uses condoms, even when one of them has sex with a guy she just met during a night out clubbing. Why should they? After all, they're on the pill. Sara, 17, says none of her friends cares much about catching diseases. "My friends think, 'This pill is going to stop me from getting pregnant'," she says, "and then they don't think about the other consequences."

These days ignorance about sexually transmitted infections, or STIs, is a common bond among teenagers and young adults the world over, and it's leading to a new global health crisis. Each year 340 million new cases of STIs are reported--a 40 percent increase since 1990, according to the World Health Organization. Most worryingly, more than half of all new cases of STIs occur in young men and women under the age of 25, of whom 41 percent report having had unprotected sex in the last year. A decade ago medical experts considered sex workers, gay men and intravenous drug users to be in the high-risk bracket. Today the group at most risk, according to new World Bank reports, are people under the age of 25.

The lack of awareness about sexual health seems a throwback to the early 1980s, before the threat of AIDS cast a death-cloud over casual sex. By the 1990s most developed nations had initiated health-education programs to teach children the dangers of disease and the importance of using condoms. The average Western teenager was raised on slogans like don't die of ignorance or posters portraying AIDS as the Grim Reaper. In recent years, though, many of these countries have let down their guard. Lower morbidity rates from AIDS and the effectiveness of antiretroviral treatment has taken some of the urgency out of the issue, while religious conservatives (especially in the United States) have made condoms controversial.

As a result, education campaigns have for the most part disappeared, and the few that do remain fail to convey the seriousness of the issue at hand. "For an hour a week my school sits us down to watch videos about spotty teens falling in love, eating bacon sandwiches and breaking out in rashes," says Richard, an 18-year-old student from South London. His friend, David, agrees that the educational messages currently being conveyed simply aren't powerful enough to be taken seriously. "We treat it as a big joke," he says. "I remember one example of a condom and a banana; everyone burst into laughter."

At the same time, sex education is only beginning to establish itself in many developing countries, even as promiscuity levels rise. China has only recently started to acknowledge its growing AIDS problem, despite being in the midst of a sexual revolution. (In the late 1980s, only 15 percent of Chinese had engaged in premarital sex; today an estimated 70 percent have.) Even though researchers warn of a "hidden epidemic" of chlamydia in China, sex education there remains practically nonexistent. Cases of syphilis in Eastern Europe--which has the fastest growing rates of STIs in the world--have risen 37-fold in the past decade, as sex education and health services have failed to keep up with the relaxation of sexual mores since the fall of the Soviet Union. When Alexandra, a 24-year-old public-relations officer from Kiev, caught chlamydia from a cheating boyfriend, she just didn't know what to do. "I didn't even know which doctor to go to," she says.

That kind of ignorance is widespread--and wide ranging. One of the most pernicious misconceptions among youths worldwide is that diseases are transmitted only through sexual intercourse. "Teenagers have this belief: 'If I don't have traditional sex, I can't get anything'," ...