Sexually transmitted infection (STI) refers to infection with a pathogen that is transmitted through blood, semen, vaginal fluids, other body fluids, or skin-to-skin contact, during oral, anal, or genital sex with an infected partner. Sexually transmitted disease (STD) refers to a recognizable disease state that has developed from an STI (1).
STIs can be caused by a number of microorganisms that vary widely in size, life cycle, susceptibility to available treatments, and the diseases and symptoms they cause.
Bacterial STIs include:
Viral STIs include:
Parasitic infections that can be sexually transmitted include:
Trichomoniasis (caused by protozoa)
Scabies (caused by mites)
Pediculosis pubis (caused by lice)
Many other infections not considered primarily to be STIs, including molluscum contagiosum, salmonellosis, shigellosis, campylobacteriosis, amebiasis, giardiasis, Zika virus infection, mpox (formerly called monkeypox), cytomegalovirus infection, hepatitis A, and hepatitis C, but can be transmitted sexually.
Because sexual activity includes close contact with skin and mucous membranes of the genitals, oropharynx, and rectum, many organisms are efficiently spread between people. Some STIs cause inflammation (eg, in gonorrhea or chlamydia) or ulceration (eg, in herpes simplex, syphilis, or chancroid), which predispose to transmission of other infections (eg, HIV).
STI rates remain high in most of the world, despite diagnostic and therapeutic advances. Worldwide in 2020, there were an estimated 374 million new curable STIs in people 15 to 49 years of age (1). In the United States in 2018, approximately 26 million new cases of STIs occurred, and about half were in people aged 15 to 24 years (2).
Factors impeding control of transmission of STIs include:
Unprotected sexual activity with multiple partners
Difficulty talking about sexual issues for both physicians and patients
Limited funding for implementing existing diagnostic tests and treatments and for developing new tests and treatments
Susceptibility to reinfection if both partners are not treated simultaneously
Incomplete treatment, which can lead to development of drug-resistant organisms
International travel, which facilitates rapid global dissemination of STIs
General references
1. World Health Organization. Sexually transmitted infections (STIs). May 29, 2025.
2. Kreisel KM, Spicknall IH, Gargano JW, et al. Sexually Transmitted Infections Among US Women and Men: Prevalence and Incidence Estimates, 2018. Sex Transm Dis. 2021;48(4):208-214. doi:10.1097/OLQ.0000000000001355
Symptoms and Signs of STIs
Symptoms and signs of STIs vary depending on the infection.
Many STIs cause genital lesions (see table Differentiating Common Sexually Transmitted Genital Lesions). In addition to lesions, STIs may cause genital discharge, itching, pain during urination or sex, and other localized or systemic symptoms.
Differentiating Common Sexually Transmitted Genital Lesions
Finding | Other Features | Cause* |
|---|---|---|
Solitary painless ulcer | Indurated, nontender or only slightly tender Relatively nontender inguinal lymphadenopathy | |
Clusters of small, painful, superficial ulcers on an erythematous base | Sometimes with vesicles In a primary episode, often inguinal lymphadenopathy and/or fever | |
Shallow, painful ulcer | Nonindurated, tender ulcers with ragged, undermined edges and an erythematous border, varying in size and often coalescing Regional adenopathy | |
Small papule or ulcer, often asymptomatic or unnoticed | Severely tender and painful adenopathy, sometimes with distal lymphedema or drainage to the skin Sometimes fever | |
Elevated nodule that ulcerates and progressively expands | Velvety, malodorous, granulating lesions No inguinal lymphadenopathy | |
Multiple, shallow ulcers | Characteristic extragenital lesions and burrows | Excoriated scabies |
Multiple, shallow lesions | Visible lice, or egg sacs (nits) attached to hair shafts | Pediculosis pubis with excoriation |
* Other causes of ulcers include oral mucous patches of secondary syphilis, erosive balanitis, gummatous ulceration of tertiary syphilis, Behçet syndrome, epithelioma, and trauma. | ||
Diagnosis of STIs
Nucleic acid amplification tests (NAATs), culture, or Gram stain of specimen from infected tissue or fluids
Blood tests
A medical history, including sexual history, is taken to assess the patient for risk factors for and symptoms of STIs. Physical examination is performed to detect signs of STIs (eg, genital ulcers or discharge, inguinal lymphadenopathy).
Diagnostic testing may include blood tests or collection of specimens from lesions or potential sites of infection for Gram staining and culture or NAATs.
STIs are diagnosed and treated in a variety of settings; sometimes, diagnostic tests are limited or unavailable, or patient follow-up is uncertain. Thus, identification of the causative organism may not occur. In such situations, diagnosis is based on clinical findings.
Treatment of STIs
Antimicrobials
Treatment of complications, if present
If possible, simultaneous treatment of sex partners
Most STIs can be effectively treated with antimicrobial medications. However, drug resistance is an increasing problem.
Clinical diagnosis alone may be used to determine treatment in the following situations:
Diagnostic testing is limited or not available.
The infection is severe and treatment cannot be delayed while waiting for a diagnostic test result.
Clinical suspicion of a particular infection is high based on history or physical examination, particularly if patient follow-up is uncertain.
Patients who are being treated for a bacterial STI should abstain from sexual intercourse until they and their sex partners have been treated and the infection has resolved. After diagnosis of an STI, sex partners should be evaluated and treated as soon as possible.
Viral STIs, especially herpes and HIV infection, usually persist for life. Antiviral medications can control but not yet cure most of these infections, and patients should be counseled about safer sex practices to prevent transmission.
Prevention of STIs
STI control depends on:
Education of health care professionals and the public
Avoidance of high-risk sexual behaviors
Adequate access to facilities and trained personnel for diagnosis and treatment
Public health programs for locating and treating recent sex partners of infected patients
Follow-up for treated patients to ensure that they have been cured
Condoms and vaginal condoms or dental dams, if used correctly, greatly decrease risk of some STIs.
Vaccines are not available for most STIs, but they are available for hepatitis A, hepatitis B, and human papillomavirus infection.
To prevent HIV infection, preexposure prophylaxis with antiretrovirals (PrEP) is an option for people who are not infected with HIV but are at high risk (eg, by having a sex partner who is infected with HIV).
Doxycycline postexposure prophylaxis (doxyPEP)
Postexposure prophylaxis to prevent syphilis, gonorrhea, and chlamydia has been evaluated in some patient populations. In 3 randomized trials, men who have sex with men (1 trial also included transgender women) taking preexposure prophylaxis (PrEP) for HIV, who received doxycycline (200 mg orally) within 72 hours after condomless sex, had significantly decreased rates of chlamydia, gonorrhea, and/or syphilis compared with those who did not receive doxycycline (Postexposure prophylaxis to prevent syphilis, gonorrhea, and chlamydia has been evaluated in some patient populations. In 3 randomized trials, men who have sex with men (1 trial also included transgender women) taking preexposure prophylaxis (PrEP) for HIV, who received doxycycline (200 mg orally) within 72 hours after condomless sex, had significantly decreased rates of chlamydia, gonorrhea, and/or syphilis compared with those who did not receive doxycycline (1–3). In contrast, a randomized trial in cisgender women did not demonstrate a decrease in STIs with use of DoxyPEP (4).
Doxycycline is a first-line medication for chlamydia and is a second-line medication for syphilis. Doxycycline is not a first-line medication for gonorrhea because of resistance, but some Neisseria gonorrhoeae remain sensitive and therefore doxyPEP may be beneficial. The Centers for Disease Control and Prevention recommends that clinicians offer doxyPEP as a preventive measure to people who are at risk of STIs and are gay or bisexual or to other men who have sex with men and transgender women unless they have contraindications to doxycycline (5).
Prevention references
1. Molina JM, Charreau I, Chidiac C, et al. Post-exposure prophylaxis with doxycycline to prevent sexually transmitted infections in men who have sex with men: an open-label randomised substudy of the ANRS IPERGAY trial. Lancet Infect Dis. 2018;18(3):308-317. doi:10.1016/S1473-3099(17)30725-9
2. Luetkemeyer AF, Donnell D, Dombrowski JC, et al. Postexposure Doxycycline to Prevent Bacterial Sexually Transmitted Infections. N Engl J Med. 2023;388(14):1296-1306. doi:10.1056/NEJMoa2211934
3. Molina JM, Bercot B, Assoumou L, et al. Doxycycline prophylaxis and meningococcal group B vaccine to prevent bacterial sexually transmitted infections in France (ANRS 174 DOXYVAC): a multicentre, open-label, randomised trial with a 2 × 2 factorial design. Lancet Infect Dis. 2024;24(10):1093-1104. doi:10.1016/S1473-3099(24)00236-6
4. Stewart J, Oware K, Donnell D, et al. Doxycycline Prophylaxis to Prevent Sexually Transmitted Infections in Women. N Engl J Med. 2023;389(25):2331-2340. doi:10.1056/NEJMoa2304007
5. Bachmann LH, Barbee LA, Chan P, et al. CDC Clinical Guidelines on the Use of Doxycycline Postexposure Prophylaxis for Bacterial Sexually Transmitted Infection Prevention, United States, 2024. MMWR Recomm Rep. 2024;73(2):1-8. Published 2024 Jun 6. doi:10.15585/mmwr.rr7302a1
