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Overview of Sexually Transmitted Diseases


Sheldon R. Morris

, MD, MPH, University of California San Diego

Last full review/revision Dec 2020| Content last modified Dec 2020
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Sexually transmitted diseases (STDs), also termed sexually transmitted infections (STIs), can be caused by a number of microorganisms that vary widely in size, life cycle, the diseases and symptoms caused, and susceptibility to available treatments.

Bacterial STDs include

Viral STDs include

Parasitic infections that can be sexually transmitted include

Many other infections not considered primarily to be STDs—including salmonellosis, shigellosis, campylobacteriosis, amebiasis, giardiasis, hepatitis (A, B, and C), Zika virus infection, and cytomegalovirus infection—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 STDs 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).

STD prevalence rates remain high in most of the world, despite diagnostic and therapeutic advances that can rapidly render patients with many STDs noninfectious. In the US, an estimated 20 million new cases of STDs 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 2018).

Factors impeding control of STDs 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 STDs

Symptoms and Signs of STDs

Symptoms and signs of STDs vary depending on the infection. Many STDs cause genital lesions (see table Differentiating Common Sexually Transmitted Genital Lesions).


Differentiating Common Sexually Transmitted Genital Lesions


Other Features


Solitary painless ulcer

Indurated, nontender or only slightly tender

Relatively nontender adenopathy

Clusters of small, painful superficial ulcers on an erythematous base

Sometimes with vesicles

Inguinal adenopathy

Shallow painful ulcer

Nonindurated, tender ulcers with ragged, undermined edges and a red 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

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

Elevated nodule that ulcerates and progressively expands

Velvety, malodorous, granulating lesions

No inguinal adenopathy

* Other causes of ulcers include mucous patches of secondary syphilis, erosive balanitis, gummatous ulceration of tertiary syphilis, Behçet syndrome, epithelioma, and trauma.

Diagnosis of STDs

  • Often clinical evaluation

  • Gram staining and culture

  • Laboratory tests

STDs are diagnosed and treated in a variety of settings; for many, diagnostic tests are limited or unavailable or patient follow-up is uncertain. Thus, identification of the causative organism is often not pursued. Often, diagnosis is based only on clinical findings.

Diagnostic testing may include Gram staining and culture or laboratory tests such as nucleic acid amplification tests (NAATs). Diagnostic testing is done more often in the following situations:

  • The diagnosis is unclear.

  • The infection is severe.

  • Initial treatment is ineffective.

  • Other reasons (eg, public health surveillance, psychosocial reasons, including extreme mental distress and depression) are compelling.

Treatment of STDs

  • Syndromic treatment

  • Sometimes antimicrobials

  • Simultaneous treatment of sex partners

Because diagnostic tests are often limited or unavailable and/or patient follow-up is uncertain, initial treatment is often syndromic—ie, directed at the organisms most likely to cause the presenting syndrome (eg, urethritis, cervicitis, genital ulcers, pelvic inflammatory disease).

Most STDs can be effectively treated with drugs. However, drug resistance is an increasing problem.

Patients who are being treated for a bacterial STD 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 STDs, especially herpes and HIV infection, usually persist for life. Antiviral drugs can control but not yet cure all of these infections.

Prevention of STDs

STD control depends on

  • Adequate 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

  • Education of health care practitioners and the public

  • Avoidance of high-risk behaviors by patients

Condoms and vaginal dams, if used correctly, greatly decrease risk of some STDs.

Vaccines are unavailable for most STDs, except for hepatitis A, hepatitis B, and human papillomavirus infection.

More Information

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

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