A health screening campaign in Liverpool has revealed more than 1,000 cases of chlamydia among the 10,000 under-25s tested.
Since November last year, under-25s across the city have been invited to special screening sessions at pubs, clubs and universities for tests, and a record number of cases of chlamydia, the most common curable sexually transmitted infection, have so far been found.
The results have prompted a warning from sexual health experts that one in 10 Liverpool people aged 15 to 24 are infected and risk infertility unless they are treated.
Susie Gardiner, senior public health practitioner, said: "One of the problems with chlamydia is that there are often no obvious signs or indications that you have it and it can take months for symptoms to appear.
"For every case that is diagnosed, however, there may be up to a further 10 cases that remain undiagnosed."
If the sexually transmitted infection is not treated it can lead to fertility and other long-term health problems, as well as the risk of passing the infection on.
"It's obvious the present messages are falling on deaf ears. A whole new rethink on sexual health education is needed and this should be a collective coordinated approach, including the children themselves and their views as to the requirements for them to listen, respond and prevent." - V Henry