Chlamydia is the most common treatable STD. It often presents no symptoms in men or women unless it leads to complications - when treatment can sometimes be too late to stop permanent damage.

In some women, infection can damage the Fallopian tubes, which conduct eggs from the ovaries to the womb. This can stop the tubes working properly and they can become completely blocked.

Chlamydia is the most common preventable cause of infertility in women. When the fallopian tubes are blocked, no pregnancy is possible naturally. One option is IVF (in-vitro fertilisation), but availability on the NHS is still patchy and has variable success rates: usually no more than 20 per cent of women end up with a baby.

Women with damaged tubes do occasionally fall pregnant, but there is an increased risk of the pregnancy developing in the tubes rather than the womb. This is called an ectopic pregnancy. The tube can split apart causing serious pain and bleeding. This is an emergency, because the bleeding can be life threatening.

Infection sometimes leads to pain in the lower abdomen that is often mistaken for some other problem rather than pelvic infection.

Men can also run into trouble. Chlamydia is the most common cause of inflammation in the testicles and sperm-conducting tubes (epididymo-orchitis) in men under 35. This causes marked pain, swelling and redness in the scrotum on the affected side, or on both sides. Chlamydia infection can also trigger joint inflammation in some men.