Chlamydia is a sexually transmitted infection caused by the bacterium Chlamydia trachomatis. It is the most common bacterial STI, it mainly affects young people 15 to 24 years of age, and is more common among women. Between 1997 and 2011, the number of infected people nearly tripled. Chlamydia is usually spread via unprotected vaginal or anal penetration or oral sex. Chlamydial infections are asymptomatic most of the time. Without having any symptoms, you can unknowingly transmit it to your sexual partners, and a mother can give it to her child at birth. Left untreated, chlamydia can cause serious damage, in particular to the female reproductive system. For this reason, regular screening is essential. Your doctor can help you decide on the frequency of your screenings. Dr. Gabrielle Landry explains here the principal causes, symptoms and treatments of chlamydial infection and the best ways to prevent it.