If a young person is diagnosed with a sexually transmitted infection, it stands to reason that they came into contact with it through someone else and may have since passed it on to others.
Check out this online service called "Let Them Know" that can help with this process, including a free SMS service (can be used anonymously).
Partner notification is crucial to preventing further transmission through testing and treatment of sexual partners.
Nearly all sexual health clinics provide a service called contact tracing. Contact tracing is a way of ensuring that sexual partners (past or present) can be contacted and also treated in an anonymous fashion. Generally it is a letter, phone call or text informing them that they someone they have had sex with has been treated for an STI and that they are being advised that they should come in for a free confidential test and treatment. Partner notification is a legal requirement and young people should be fully briefed about this process before undergoing an STI screen themselves to avoid a rude shock should any tests come back positive.
You can explain contact tracing to a young person with these helpful videos.
What's involved in a sexual health check in Australia Highly recommended video. The journey of a young person getting chlamydia testing and treatment and notifying his ex-partner (with excellent explanations along the way).
The Phone Call Robert calls past sexual partners to tell them about his diagnosis of Chlamydia.