Sexually Transmitted Infections

Sexually transmitted infections (STIs) are infections transmitted predominantly through sexual contact, caused by bacteria, viruses, and protozoa. In women they frequently involve the lower genital tract and, when caused by organisms such as Chlamydia trachomatis and Neisseria gonorrhoeae, can ascend to cause upper-tract disease with reproductive consequences.

Definition

Sexually transmitted infections are infections spread chiefly through sexual contact, encompassing bacterial (for example chlamydia, gonorrhoea, syphilis), protozoal (trichomoniasis), and viral (for example HPV, HSV, HIV) pathogens.

Scope

The entry covers STIs as a category within gynecologic infections: the main groups of pathogens, the distinction between curable bacterial and protozoal infections and chronic viral ones, common syndromes in women (cervicitis, urethritis, vaginitis, ulcerative disease), and their public-health scale. It is an orienting reference, not a manual for screening, diagnosis, or treatment of any individual.

Core questions

• Which pathogens account for the major curable and incurable STIs?
• What genital syndromes in women do STIs commonly produce?
• How do cervical STIs relate to ascending upper-tract infection?
• Why are many STIs asymptomatic, and what does that mean for transmission and complications?

Key concepts

• Curable versus chronic (viral) STIs
• Chlamydia trachomatis and Neisseria gonorrhoeae
• Trichomonas vaginalis
• Syphilis (Treponema pallidum)
• Asymptomatic infection and silent transmission
• Syndromic versus aetiological diagnosis
• Reproductive and perinatal complications

Mechanisms

STIs are acquired through contact of mucosal or skin surfaces during sexual activity, allowing pathogens to colonise or invade the genital, anal, or oropharyngeal mucosa. In women, bacterial pathogens commonly infect the columnar epithelium of the cervix, producing cervicitis, and from there organisms such as Chlamydia trachomatis and Neisseria gonorrhoeae can ascend to cause pelvic inflammatory disease (Brunham, 2015 — see related entry). Protozoal infection by Trichomonas vaginalis produces vaginitis (Swygard, 2004). Many infections are asymptomatic, which sustains transmission within sexual networks and allows complications to develop before the infection is recognised.

Clinical relevance

STIs are clinically and epidemiologically important because they cause genital symptoms, can lead to serious reproductive complications including infertility and ectopic pregnancy when the upper tract is involved, can be transmitted to infants during pregnancy or delivery, and certain infections increase susceptibility to HIV. This entry describes the category and its consequences for educational orientation; it is not a basis for individual screening or treatment decisions.

Epidemiology

The four main curable STIs — chlamydia, gonorrhoea, trichomoniasis, and syphilis — account for an estimated hundreds of millions of new infections each year worldwide among adults, with trichomoniasis and chlamydia being especially common (Newman, 2015; Rowley, 2019). The burden falls heavily on women of reproductive age and on lower- and middle-income settings.

Evidence & guidelines

Global prevalence and incidence have been estimated through systematic review and surveillance reporting (Newman, 2015; Rowley, 2019), and the CDC Sexually Transmitted Infections Treatment Guidelines provide a widely used reference for the syndromes and their management (Workowski, 2021). This entry summarises such sources for orientation and does not reproduce their recommendations.

Seminal works

• newman-2015
• rowley-2019
• workowski-2021

Frequently asked questions

What are the most common sexually transmitted infections?

Among the four main curable infections, trichomoniasis and chlamydia are the most common globally, followed by gonorrhoea and syphilis; chronic viral infections such as human papillomavirus, herpes simplex virus, and HIV are also widespread.

Why are many STIs described as asymptomatic?

A large share of infections, especially chlamydia in women, produce few or no symptoms, so they can be transmitted to others and cause complications such as pelvic inflammatory disease before they are noticed.