Chlamydia ia one of the most common sexually transmitted infections (STIs). If untreated it can become painful and cause serious health problems such as infertility. Chlamydia is caused by bacteria, which are found in semen and vaginal fluid and are easily passed from one person to another through sexual contact. It is particularly common in younger people but anyone who is sexually active can get it. Both men and women can have, and pass on Chlamydia. Often there are no symptoms or they may be so mild you do not know you have it.