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, symptoms, and susceptibility to available treatments.

Bacterial STDs include syphilis, gonorrhea, chancroid, lymphogranuloma venereum, granuloma inguinale, and chlamydial, mycoplasmal, and Ureaplasma infections.

Viral STDs include genital and anorectal warts, genital herpes (see Herpesviruses: Mucocutaneous infection), molluscum contagiosum (see Viral Skin Diseases: Molluscum Contagiosum), and HIV infection (see Human Immunodeficiency Virus (HIV)).

Parasitic infections that can be sexually transmitted include trichomoniasis (caused by protozoa), scabies (caused by mites—see Parasitic Skin Infections: Scabies), and pediculosis pubis (caused by lice—see Parasitic Skin Infections: Pubic lice).

Many other infections not considered primarily to be STDs—including salmonellosis, shigellosis, campylobacteriosis, amebiasis, giardiasis, hepatitis (A, B, and C), 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. Inflammation or ulceration caused by some STDs (eg, herpes, chancroid) predisposes to transmission of others (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. 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 new therapy and research
• Susceptibility to reinfection if both partners are not treated simultaneously
• Incomplete treatment, which leads to development of drug-resistant organisms
• International travel, which facilitates rapid global dissemination of STDs

Symptoms and signs vary depending on the infection. Many STDs cause genital lesions (see Table 1: Sexually Transmitted Diseases (STD): Differentiating Common Sexually Transmitted Genital Lesions).

Table 1
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Differentiating Common Sexually Transmitted Genital Lesions Finding Other Features Cause* Solitary ulcer Indurated, painless or only slightly tender Relatively nontender adenopathy Syphilitic chancre Clusters of small, superficial ulcers on an erythematous base Painful, sometimes with vesicles Inguinal adenopathy Herpes simplex virus infection Shallow ulcer Nonindurated, painful ulcers with ragged, undermined edges and a red border, varying in size and often coalescing Buboes Chancroid Small papule or ulcer, often asymptomatic or unnoticed Severely tender and painful buboes, sometimes with distal lymphedema or drainage to the skin Fever possible Lymphogranuloma venereum Multiple, shallow lesions Systemic symptoms (eg, fever, rash, adenopathy) Characteristic extragenital lesions and burrows Primary HIV infection Excoriated scabies Multiple, shallow lesions Presence of lice — Pediculosis pubis Elevated lesion Velvety, malodorous, granulating lesions No inguinal adenopathy Granuloma inguinale *Other causes of ulcers include mucous patches of secondary syphilis, erosive balanitis, gummatous ulceration of tertiary syphilis, Behçet's syndrome, epithelioma, and trauma.

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, and 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). Diagnostic testing is done more often when the diagnosis is unclear, when the infection is severe, when initial treatment is ineffective, or when other reasons (eg, public health surveillance, psychosocial reasons, including extreme mental distress and depression) are compelling.

STD control depends on

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

Last full review/revision November 2008 by J. Allen McCutchan, MD, MSc