Chlamydia trachomatis is one of the most common sexually transmitted infections. It is caused by a microbe that, like a virus, can only reproduce inside one of your own body cells (so is difficult to grow in a laboratory) and is too small to be seen under a light microscope. It is unlike a virus, however, in that (like bacteria) it is sensitive to certain antibiotics.

What symptoms will you notice?
Chlamydia produces no obvious symptoms in at least 6 out of 10 people infected. When it does produce symptoms, Women may notice a mucky vaginal discharge but this is not always present and often it remains undiagnosed, or is picked up during a routine screen or because her partner develops symptoms.

Men may notice discharge from the penis, or discomfort on passing urine. The discharge is usually present early in the morning and clears during the day. Staining of the underwear with a mucus or slightly pus-stained discharge is another telltale sign.

How is Chlamydia diagnosed?
Using immune tests that detect specific Chlamydia proteins or genetic material. Suspected infection is often treated before it is confirmed, as the test takes several days to provide a result. Suspicion of Chlamydia is aroused if:

• A partner has tested positive for Chlamydia
• More pus cells than normal are seen when examining a swab taken from the male urethra or female cervix
• Threads (caused by clumps of cells) are seen in a male sample of urine

How is Chlamydia treated?
Once diagnosed or suspected, Chlamydia is treated with antibiotics such as oxytetracycline, doxycycline, erythromycin or azithromycin. It's important not to resume sexual activity until treatment has finished and you're given the all clear, otherwise infection can be passed to and fro between you and your partner (who must also be treated).

What happens if Chlamydia goes untreated?
In women, Chlamydia can lead to severe lower abdominal pain due to pelvic inflammatory disease (PID). This occurs when infection spreads to the Fallopian tubes and causes them to become swollen and sometimes blocked with scar tissue. Unfortunately, after a single attack of PID, around one in eight women have difficulty conceiving naturally - the risk increases with further attacks.

If you do get pregnant after PID, there is an increased risk of ectopic pregnancy (where the fertilised egg gets stuck in a Fallopian tube on its way down to the womb. It then starts developing in the tube rather than in the womb, which can burst to cause severe abdominal pain). This is a dangerous condition if not diagnosed and treated early.

A baby can also pick up Chlamydia during childbirth and develop a form of conjunctivitis, which must be treated to prevent damage to the cornea of the eye.

Chlamydia can spread to cause inflammation of the prostate gland or testicles (epididymo-orchitis) in men. It can also trigger an immunological reaction called Reiter's syndrome in one per cent of affected men. Reiter's syndrome is diagnosed by the presence of inflammation of the urethra (urethritis), bilateral conjunctivitis (and sometimes uveitis - inflammation of the lining of the eye, including the iris) plus inflammation of one or more joints (arthritis). Most men with Reiter's syndrome have recently had sex with someone new, which was followed by urethral inflammation and discharge.

Treatment of Reiter's arthritis is with painkillers and anti-inflammatory drugs. The inflammation - which may be due to Chlamydia - is treated with antibiotics. Most first attacks resolve within two to six months, but recovery can be as long as a year. Unfortunately, the arthritis flares up again in a third of cases.

If you're worried you have an STI or want to find out more, check out the symptoms, diagnosis and treatments for:

• Genital herpes (HSV)
• Genital warts
• Gonorrhoea
• HIV and AIDS
• Pubic lice
• Scabies
• Syphilis
• Trichomonas