**1. Introduction**

*Chlamydia trachomatis* (CT) is the most frequent bacterial agent causing sexually transmitted infections (STIs) worldwide. According to the latest World Health Organization (WHO) report, approximately 100 million new infections occur annually [1]. According to the Centre for Disease Control (CDC), around 1.4 million new CT cases were reported in 2013, only in the United States [2].

In this chapter, we briefly present the current knowledge about the cell biology of the bacteria, reviewing the mechanisms of establishment of CT intracellular niche, the inducers of persistent

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infections, and the pathogen factors that may be involved in the damage of female reproductive tract. Then, we analyze the host factors that may contribute to the development of infertility, mainly immune response and genetic predisposition, hormonal status, and sexual behavior. Undiagnosed and untreated infections, repeat and persistent infections, and coinfections are likely responsible for the detrimental sequelae on woman fertility of CT pathogenesis.

This chapter specially focuses on the consequences of chronic diseases after CT infections, mainly pelvic inflammatory disease, tubal infertility, and adverse pregnancy outcome, which are of therapeutic interest in the reproduction field.
