Chlamydia is by far the most common STI in the UK. It's the leading preventable cause of female infertility in the UK because the infection can spread to the fallopian tube and ovaries. The bad news is that 7 out of 10 women and 5 out of 10 men won't show any symptoms. That means without a test, you might not know whether you have it.
Observable symptoms in women - if they do occur - can be a yellow vaginal discharge, vaginal bleeding between periods and a burning sensation when urinating. If and when the infection spreads to the fallopian tube and ovaries, Chlamydia might also cause a fever and discomfort in the lower abdomen.
Observable symptoms in men - if they do occur - tend to be a white/clear penile discharge and/or a burning sensation when urinating. If the infection spreads, Chlamydia can cause painful swelling in the testicles or scrotum.
In the worst case scenario Chlamydia can eventually cause a form of arthritis called SARA in both men and women. SARA will give you painful joints.
The latest government data - for 2004 - shows that in England rates of Chlamydia infection in England were highest in London, Yorkshire, Humberside and the North West. In 2004 over 100,000 cases were diagnosed in the UK.
If you don't use a condom when you have vaginal or anal sex with someone who already has Chlamydia, there's a good chance you'll catch it. The same goes for when a condom breaks, even if a man hasn't ejaculated. Catching it from oral sex is possible, but much less common.
Chlamydia can be diagnosed from a urine sample or from a physical swab from the part of your body that might be infected. Treatment involves a doctor prescribing antibiotics and treatment tends to be very effective at getting rid of the infection.