Infertility affects approximately 15% of couples of reproductive age worldwide, with male factors contributing to nearly 40%–50% of cases. Because semen quality is a fundamental indicator of male fertility, understanding its determinants is essential for clinical assessment. In this retrospective cohort study involving 131 183 patients, we compared semen parameters between infertile men and healthy controls and explored the relationships among age, infectious microorganisms, and semen quality. Our analysis demonstrated that infertile men had significantly poorer semen quality than controls did. Age also played a critical role, with the lowest risk of male infertility observed in the 25–29 years age group; across all age groups, risk followed a U-shaped pattern. Furthermore, the presence of antisperm antibodies, Neisseria gonorrhea, Chlamydia trachomatis, Ureaplasma urealyticum, and/or Mycoplasma genitalium was associated with reduced semen quality and increased infertility risk. In contrast, although herpes simplex virus II was detected in some semen samples, its overall prevalence was low, and no significant impact on semen parameters was detected. These findings emphasize the complex interplay among age, infection, and immune factors in shaping male reproductive potential. They also highlight the need for targeted prevention and treatment strategies to mitigate the burden of male infertility and to improve future therapeutic outcomes.

Keywords: antisperm antibodies; Chlamydia trachomatis; male fertility; Mycoplasma genitalium; Neisseria gonorrhea; semen;
Ureaplasma urealyticum