Chlamydia is a sexually transmitted infection (STI) caused by the bacterium chlamydia trachomatis. In the UK, the number of new diagnoses has been steadily increasing each year since the mid-1990s, and it has now become the most commonly diagnosed STI.

Between 2004 and 2005, the number of confirmed cases of chlamydia rose by 5%, from 104,733 to 109,958. Sexually active women aged under-25, have a 1 in 10 chance of getting chlamydia, and men who are aged between 20 and 30 are most at risk of becoming infected.

As chlamydia sometimes has no symptoms, in both men and women, it can often go undiagnosed, unless it leads to complications. However, once the infection has been diagnosed, it is treatable.