Merck Manual

Please confirm that you are a health care professional

honeypot link

Overview of Sexually Transmitted Infections


Sheldon R. Morris

, MD, MPH, University of California San Diego

Last full review/revision Dec 2020| Content last modified Sep 2022
Click here for Patient Education
Topic Resources

Sexually transmitted infections (STIs), also termed sexually transmitted diseases or STDs, 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 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 , 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 that can rapidly render patients with many STIs noninfectious. In the US, an estimated 20 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 2018).

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 Differentiating Common Sexually Transmitted Genital Lesions Differentiating Common Sexually Transmitted Genital Lesions Differentiating Common Sexually Transmitted Genital Lesions ).


Diagnosis of STIs

  • Often clinical evaluation

  • Gram staining and culture

  • Laboratory tests

STIs 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 STIs

  • 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 STIs can be effectively treated with drugs. However, drug resistance is an increasing problem.

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 drugs can control but not yet cure all of these infections.

Prevention of STIs

More Information

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

Click here for Patient Education
NOTE: This is the Professional Version. CONSUMERS: Click here for the Consumer Version
quiz link

Test your knowledge

Take a Quiz!