Sexually transmitted infections (STIs) are a category of infections of the urogenital tract that are transmitted through sexual contact. These infections may be bacterial, viral, or parasitic and may manifest differently in men and women. The use of condoms and other barrier methods, abstinence from sexual intercourse, and monogamous sexual relationships help prevent contraction of STIs. Management options depend on the etiology of the infection. In pregnant women, STIs are a risk factor for preterm premature rupture of membranes, which results in early labor and infants born prematurely. This entry is divided into three sections: bacterial, viral, and parasitic STIs. These sections include descriptions of the cause, epidemiology, symptoms, diagnostic testing, and treatment for each type of infection.