Whereas obstructive azoospermia caused by the classic venereal diseases, such as gonorrhea, was formerly a common reason for male infertility among European men (i.e., before antibiotics became generally available), nowadays Chlamydia trachomatis and Mycoplasma spp, and particularly Ureaplasma urealyticum, as well as gram-negative bacteria typical of urogenital infections are the predominant microorganisms causing an infection of the seminal ducts (Paavonen and Eggert-Kruse 1999; Weidner et al. 1999; Schneede et al. 2003). The infections are manifested along the seminal ducts (see Fig. 3.5) in the forms of urethritis, prostatitis, vesiculitis or epididymitis, but can often present without clinical symptoms. For orchitis see Sect. 13.5.