**4. Conclusions**

Chlamydial infections are one of the most prevalent infectious diseases reported worldwide. The spectrum of diseases varies from asymptomatic silent infection to severe disease affecting ocular, genito-urinary, and pulmonary systems. Without proper management, the infections might progress and result in blindness, infertility, sepsis, or death. Antibacterial medications such as tetracyclines, macrolides, and quinolones, which have been used for many years to treat a variety of infections that are also effective for human infections caused by *C. pneumoniae*, *C. trachomatis*, and *C. psittaci*. Although persistent and heritable antibacterial resistance is common in some *Chlamydia* spp. such as *C. suis*, genotypic stable resistance has not been reported in human chlamydial infections. Phenotypic antibacterial resistance might result from the persistence of microorganisms or heterotypic resistance, whereas antibiotic failure might result from non-adherence to medication, re-infection, or choice of regimens with lower success. The laboratory diagnosis of chlamydial infections might be challenging in many centers and patient follow-up cannot be assured on some occasions; therefore, in case of doubt of chlamydial infection early presumptive treatment with recommended regimens, proper follow-up of the patients, prevention of re-infection via patient and partner counseling and protection of susceptible populations by the implementation of screening programs in high-risk patients should be established.
