The main objective of the study is to determine whether early screening and treating young women (<25 years of age) for genital Chlamydia Trachomatis (Ct) infection reduces the cumulative incidence of pelvic inflammatory disease (PID) over 24 months. As secondary objectives, the study aims To determine the baseline prevalence and the incidence of Ct infection; To improve knowledge on natural history of Ct infection in young women such as the rate and timing of progression to PID (at the beginning of the infection, at the end, throughout the course of infection), as well as the incidence of reinfections with Ct; To investigate the relation between host immuno-genetic factors and the clearance, persistence and development of late complications (PID) as an explanation for the inter-individual heterogeneity in the susceptibility to and course of Ct infection.