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Overview of Sexually Transmitted Infections


Sheldon R. Morris

, MD, MPH, University of California San Diego

Reviewed/Revised Jan 2023
Topic Resources

Sexually transmitted infection (STI) refers to infection with a pathogen that is transmitted through blood, semen, vaginal fluids, or other body fluids 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 Reference Sexually transmitted infection (STI) refers to infection with a pathogen that is transmitted through blood, semen, vaginal fluids, or other body fluids during oral, anal, or genital sex with... read more ). 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

Many other infections not considered primarily to be STIs—including salmonellosis Overview of Salmonella Infections The genus Salmonella is divided into 2 species, S. enterica and S. bongori, which include > 2500 known serotypes. Some of these serotypes are named. In such cases, common... read more , shigellosis Shigellosis Shigellosis is an acute infection of the intestine caused by the gram-negative Shigella species. Symptoms include fever, nausea, vomiting, tenesmus, and diarrhea that is usually bloody... read more , campylobacteriosis Campylobacter and Related Infections Campylobacter infections typically cause self-limited diarrhea but occasionally cause bacteremia, with consequent endocarditis, osteomyelitis, or septic arthritis. Diagnosis is by culture... read more , amebiasis Amebiasis Amebiasis is infection with Entamoeba histolytica. It is acquired by fecal-oral transmission. Infection is commonly asymptomatic, but symptoms ranging from mild diarrhea to severe dysentery... read more , giardiasis Giardiasis Giardiasis is infection with the flagellated protozoan Giardia duodenalis (G. lamblia, G. intestinalis). Infection can be asymptomatic or cause symptoms ranging from intermittent... read more , hepatitis Overview of Acute Viral Hepatitis Acute viral hepatitis is diffuse liver inflammation caused by specific hepatotropic viruses that have diverse modes of transmission and epidemiologies. A nonspecific viral prodrome is followed... read more (A, B, and C), Zika virus infection Zika Virus (ZV) Infections The Zika virus is a mosquito-borne flavivirus that is antigenically and structurally similar to the viruses that cause dengue, yellow fever, and West Nile virus. Zika virus infection is typically... read more , monkeypox (mpox) Mpox (Monkeypox) Mpox (monkeypox)is caused by the monkeypox virus, an orthopoxvirus structurally related to the smallpox virus. Patients present with a vesicular or pustular rash that may be painful and... read more Mpox (Monkeypox) , and cytomegalovirus infection Cytomegalovirus (CMV) Infection Cytomegalovirus (CMV, human herpesvirus type 5) can cause infections that have a wide range of severity. A syndrome of infectious mononucleosis that lacks severe pharyngitis is common. Severe... read more —can be transmitted sexually.

Because sexual activity includes close contact with skin and mucous membranes of the genitals, mouth, and rectum, many organisms are efficiently spread between people. Some STIs cause inflammation (eg, in gonorrhea or chlamydial infection) or ulceration (eg, in herpes simplex, syphilis, or chancroid), which predispose to transmission of other infections (eg, HIV).

STI prevalence rates remain high in most of the world, despite diagnostic and therapeutic advances. In the US, over 25 million new cases of STIs occur each year; about half occur in people aged 15 to 24 years (see also Centers for Disease Control and Prevention [CDC]: Sexually Transmitted Disease Surveillance 2020).

Factors impeding control of STIs include

  • Unprotected sexual activity with multiple partners

  • Difficulty talking about sexual issues for both physicians and patients

  • Inadequate 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


Symptoms and Signs of STIs

Symptoms and signs of STIs vary depending on the infection. Many STIs cause genital lesions (see table ).


Diagnosis of STIs

  • History and physical examination

  • Gram staining and culture or nucleic acid amplification tests (NAATs)

  • Blood tests

A medical history, including sexual history, is taken to assess for risk factors for STIs. Physical examination is performed if symptoms are associated with potential STIs.

Diagnostic testing may include blood tests or collection of specimens from lesions or potential sites of infections 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

  • Usually 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 the infection has been eliminated from them and their sex partners. Sex partners should be evaluated and treated simultaneously.

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 practitioners and the public

  • Avoidance of high-risk behaviors by patients

  • Adequate access to facilities and trained personnel for diagnosis and treatment

  • Public health programs for locating and treating recent sex partners of 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. For some infections and patients, pre-exposure prophylaxis with immunizations or medications is appropriate.

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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