Infectious agents can impair various important human functions, including reproduction. Bacteria, fungi, viruses and parasites are able to interfere with the reproductive function in both sexes. Infections of male genito-urinary tract account for about 15% of the case of male infertility. Infections can affect different sites of the male reproductive tract, such as the testis, epididymis and male accessory sex glands. Spermatozoa themselves subsequently can be affected by urogenital infections at different levels of their development, maturation and transport. Among the most common microorganisms involved in sexually transmitted infections, interfering with male fertility, there are the Chlamydia trachomatis and Neisseria gonorrhoeae. Less frequently male infertility is due to non-sexually transmitted epididymo-orchitis, mostly caused by Escherichia coli. In female, the first two microorganisms are certainly involved in cervical, tubal, and peritoneal damage, while Herpes simplex cervicitis is less dangerous. The overall importance of cervical involvement is still under discussion. Tubo-peritoneal damage seems to be the foremost manner in which microorganisms interfere with human fertility. C. trachomatis is considered the most important cause of tubal lacerations and obstruction, pelvic inflammatory disease (PID) and adhesions. N. gonorrhoeae, even though its overall incidence seems to decline, is still to be considered in the same sense, while bacterial vaginosis should not be ignored, as causative agents can produce ascending infections of the female genital tract. The role of infections, particularly co-infections, as causes of the impairment of sperm quality, motility and function needs further investigation. Tropical diseases necessitate monitoring as for their diffusion or re-diffusion in the western world.