The number of new cases of sexually transmitted infections (STIs) diagnosed in the UK increased to almost half a million last year, according to figures.
Almost 12,000 more cases were diagnosed in sexual health clinics last year than in 2008, with 482,696 cases found overall, the Health Protection Agency (HPA) said.
Young women often "lack the skills and confidence to negotiate safer sex" and so are "particularly vulnerable", the HPA said.
Some 73% of new cases of gonorrhoea found in women were among those under the age of 25, with 68% of genital warts also found in this age bracket. In all, the under-25s accounted for two-thirds of new STI cases found in women. Nearly nine out of 10 (88%) of new cases of the most common STI in the UK, chlamydia, were found in women in the same age group.
Chlamydia cases among both sexes increased by 7% across all age groups between 2008 and 2009, while cases of gonorrhoea rose 6%.
Genital herpes cases were up by 5% while genital warts cases stabilised with a 0.3% drop. Syphilis cases fell by 1%.
While more chlamydia testing in the community - and more sensitive tests for other STIs - accounted for much of the rise, the HPA said it was clear many people were still having unsafe sex.
Young adults and gay men are the groups most worrying the HPA.
Re-infection is also an issue, with at least 11% of 16 to 19-year-old women in England becoming re-infected with an STI within one year of being treated for a previous one.
The same is true for young men, with 12% of 16 to 19-year-olds becoming re-infected within one year.
Dr Gwenda Hughes, Head of the HPA's STI section, said: "Re-infection is a worrying issue - the numbers we're seeing in teenagers are of particular concern as this suggests teenagers are repeatedly putting their own, as well as others, long-term health at risk from STIs."