This is a bacterium Chlamydia trachomatis. It is the most common STI and young people are, for unknown reasons, especially at risk.
What are the symptoms?
This is a difficult one. Often symptoms don't show at all and this is why it can be potentially very damaging. It is usually only found by specific screening at a sexual health (GUM) clinic. If there are any symptoms they will be a vaginal discharge, pain in the pelvic area during sex or, as with men, pain when urinating.
Is there treatment available and what is it?
Yes - through antibiotics
Are there long term effects?
Not if it is caught early enough. However it can cause infertility if not caught in time - and, if passed on from a mother to her baby, the effects can be terrible.
If your boyfriend or girlfriend falls silent, something's up.
Explain that you want to be 'let in' - then it can be good to give them space, tell them to call and let them come back to you