Casual sexual behaviour, often fuelled by alcohol, is causing an alarming rise in sexually transmitted infections among teenage girls and young men, the Health Protection Agency said yesterday.

Gay men are also contracting increasing numbers of infections, which suggests the fear of HIV/Aids has decreased and that more people are having unsafe sex.

The overall number of infections went up last year by 2%, from 368,341 to 376,508, which the HPA says may suggest a slowdown in what had appeared to be a rapid rise over the past few years. But there are real concerns about the behaviour of young people and gay men. Among teenage girls aged 16 to 19 the numbers catching genital herpes - an unpleasant sexual infection which is treatable but never completely cured - are up by 16%.

"We are really quite concerned about genital herpes," said Gwenda Hughes of the HPA's centre for infections. "It is a lifelong infection and not curable. It is treatable but people can experience severe recurrences. It is also associated with quite a lot of stigma."

Pat Troop, chief executive of the HPA, said it was hard to get the safe sex message through to young people in a society where cultural messages were unhelpful.

"You don't see people in films pulling out a condom," she said. "The images that bombard young people don't make it easy for them. It was just the same with smoking - young people used to see people smoking on television and it didn't help them give up when all the people they might admire were smoking."

Behaviour surveys over the last 10 years have shown that young people are having more partners, more casual sex and are less likely to use condoms than before.

Government campaigns are trying to use the voices of young people talking about sexual diseases they have picked up, in adverts on commercial radio, to get the message across. "Young people listen to each other," Professor Troop said. "It is a matter of having a really sustained programme involving young people and easy access to services where they can get advice."

Several types of sexually transmitted infections have increased in the last year - gonorrhoea by 3%, chlamydia by 19% and genital warts by 4%. "We are seeing increasing levels of unsafe sexual behaviour among gay men. It may mean gay men are placing themselves at increasing risk of HIV infection," said Dr Hughes.