Sexually transmitted infections (STIs) are a common cause of human infertility worldwide. Being promiscuous greatly increases the risk of acquiring STIs.
The bacteria Chlamydia trachomatis and Neisseria gonorrhoeae are the major causes of pelvic inflammatory disease (PID), which in untreated women results in tubal factor infertility in 10%–40% of the cases and increases the probability of ectopic pregnancy by more than six times. These two pathogens are responsible for more than 200 million new infections per year across the world; with an estimated probability of PID up to 10%–25% in both infections, the global burden of new cases of infertility due to these two pathogens may exceed 2 million per year.
N. gonorrhoeae and, to a lesser extent, C. trachomatis have also been associated with reduced male fertility, and both bacteria have been linked to increased risk of perinatal complications. Treponema pallidum, the bacterial agent of syphilis, causes about 10 million new infections per year, and has a dramatic impact on pregnancy, with about one-third of untreated cases resulting in perinatal death (stillbirth or neonatal mortality) and another third in congenital infection.
In addition to these major bacterial sexually transmitted diseases (STDs), a number of other bacteria that can potentially be transmitted by sexual contact (e.g., Gardnerella vaginalis, Mycoplasma hominis and Mycoplasma genitalium, Ureaplasma urealyticum) have been associated with bacterial vaginosis, a frequent condition of disturbed vaginal microflora, which may increase the risk of PID and infertility in women.
The sexually transmitted unicellular eukaryotic parasite Trichomonas vaginalis infects more than 270 million people per year and increases the probability of pre-term birth, PID, female tubal, and male infertility. Finally, some sexually transmitted viruses have also been implicated in reproductive health: genital herpes (caused by human herpesvirus types 1 and 2) may cause miscarriage and/or pregnancy complications and is associated with reduced fertility in both sexes; human papillomaviruses may cause reduced sperm motility and an increased risk of abortion; human immunodeficiency virus infection adversely affects sperm quality.