Infection is usually asymptomatic. Sexually active people aged under 20 in the UK are the group most likely to have a positive result if tested. This is probably because this group changes partners more frequently. However, there also appear to be immunological factors which make infection more persistent in the young. Transmission occurs through vaginal, rectal or oral sex. It can also be vertically transmitted. Untreated chlamydia infection can result in complications including pelvic inflammatory disease (potentially leading to infertility or ectopic pregnancy), sexually acquired reactive arthritis and epididymo-orchitis. There is controversy over important questions such as the likelihood of complications developing and, hence, what sort of control measures are appropriate. Some countries, for example England, have set up screening programmes while others, such as Scotland, have elected not to. [With external links to the evidence base]