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Overview of Sexually Transmitted Infections (STIs)


Sheldon R. Morris

, MD, MPH, University of California San Diego

Full review/revision Jan 2021 | Modified Sep 2022
Topic Resources

Sexually transmitted (venereal) diseases are infections that are typically, but not exclusively, passed from person to person through sexual contact.

  • Sexually transmitted infections may be caused by bacteria, viruses, or protozoa.

  • Some infections can also be spread through kissing or close body contact.

  • Some infections may spread to other parts of the body, sometimes with serious consequences.

  • Using condoms can help prevent these infections.

  • Most sexually transmitted infections can be effectively treated with drugs.

Sexual intercourse provides an easy opportunity for organisms to spread (be transmitted) from one person to another because it involves close contact and transfer of genital and other body fluids.

Sexually transmitted infections (STIs), also called sexually transmitted diseases or STDs, are relatively common. For example, an estimated 20 million new cases of STIs occur each year in the United States. About half of the new cases occur in people age 15 to 24 years. Over 580,000 new cases of gonorrhea and over 1.8 million chlamydial infections were reported in the United States 2018, and even more probably occur—making gonorrhea and chlamydial infections the two most common STIs.

Several factors make prevention of STIs difficult. They include the following:

  • Unprotected sexual activity with several partners (whose names may be unknown and who thus may be difficult to find)

  • Reluctance to talk about sexual issues with a health care practitioner

  • The need for more funding to identify and treat as many infected people as possible and to develop better diagnostic tests and treatments for STIs

  • The need to treat both sex partners simultaneously if one of them is infected

  • Incomplete treatment, which can lead to development of organisms that are resistant to drugs

  • International travel, which enables STIs to be rapidly spread worldwide

Causes of STIs


Many infectious organisms—from tiny viruses, bacteria, and parasites to visible insects (such as lice)—can be spread through sexual contact. Some infections that can be transmitted during sexual activity are often spread in other ways. Thus, they are not typically considered STIs. These infections include hepatitis Overview of Acute Viral Hepatitis Acute viral hepatitis is inflammation of the liver, generally meaning inflammation caused by infection with one of the five hepatitis viruses. In most people, the inflammation begins suddenly... read more A, B, and C and infections of the digestive tract (which cause diarrhea), such as Salmonella infections Salmonella Infections The gram-negative bacteria Salmonella typically cause diarrhea and sometimes cause a more serious infection, typhoid fever. People are usually infected when they eat contaminated food... read more , Campylobacter infections Campylobacter Infections Several species of the gram-negative bacteria Campylobacter (most commonly Campylobacter jejuni) can infect the digestive tract, often causing diarrhea. People can be infected... read more , shigellosis Shigellosis Shigellosis is infection by the gram-negative bacteria Shigella. It results in watery diarrhea or dysentery (the frequent and often painful passage of small amounts of stool that contain... read more , giardiasis Giardiasis Giardiasis is an infection of the small intestine caused by the single-celled protozoan parasite Giardia. The main symptoms are abdominal cramping and diarrhea. People may have abdominal... read more , and amebiasis Amebiasis Amebiasis is an infection of the large intestine and sometimes the liver and other organs that is caused by the single-celled protozoan parasite Entamoeba histolytica, an ameba. The amebas... read more .


Although STIs usually result from having vaginal, oral, or anal sex with an infected partner, genital penetration is not necessary to spread an infection. Some STIs can also be spread in other ways, including

Symptoms of STIs

Symptoms of STIs vary greatly, but the first symptoms usually involve the area where the organisms entered the body. For example, sores may form in the genital area or mouth. There may be a discharge from the penis or the vagina, and urination may be painful.

Some of the symptoms increase the risk of getting other infections (such as HIV infection Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and can cause acquired immunodeficiency syndrome (AIDS). HIV is transmitted... read more Human Immunodeficiency Virus (HIV) Infection ). For example, having irritated skin (inflammation, as occurs in gonorrhea Gonorrhea Gonorrhea is a sexually transmitted infection caused by the bacteria Neisseria gonorrhoeae, which infect the lining of the urethra, cervix, rectum, and throat or the membranes that cover... read more Gonorrhea or chlamydial infection Chlamydial and Other Nongonococcal Infections Chlamydial infections include sexually transmitted infections of the urethra, cervix, and rectum that are caused by the bacteria Chlamydia trachomatis. These bacteria can also infect... read more Chlamydial and Other Nongonococcal Infections ) or sores (as occurs in herpes Herpes Simplex Virus (HSV) Infections Herpes simplex virus infection causes recurring episodes of small, painful, fluid-filled blisters on the skin, mouth, lips (cold sores), eyes, or genitals. This very contagious viral infection... read more Herpes Simplex Virus (HSV) Infections , syphilis Syphilis Syphilis is a sexually transmitted infection caused by the bacteria Treponema pallidum. Syphilis can occur in three stages of symptoms, separated by periods of apparent good health. It... read more Syphilis , or chancroid Chancroid Chancroid is a sexually transmitted infection caused by the bacteria Haemophilus ducreyi, which causes painful genital sores. In developed countries, chancroid is rare. In 2018, only... read more ) makes it easier for other infectious organisms to enter the body.


When STIs are not diagnosed and treated promptly, some organisms can spread through the bloodstream and infect internal organs, sometimes causing serious, even life-threatening problems. Such problems include

In women, some organisms that enter the vagina can infect other reproductive organs. The organisms can infect the cervix (the lower part of the uterus that forms the end of the vagina), enter the uterus, and reach the fallopian tubes and sometimes the ovaries (see figure Pathway From the Vagina to the Ovaries Pathway From the Vagina to the Ovaries Pathway From the Vagina to the Ovaries ). Damage to the uterus and fallopian tubes can result in infertility or a mislocated (ectopic Ectopic Pregnancy Ectopic pregnancy is attachment (implantation) of a fertilized egg in an abnormal location, such as the fallopian tubes. In an ectopic pregnancy, the fetus cannot survive. When an ectopic pregnancy... read more ) pregnancy. The infection may spread to the membrane that lines the abdominal cavity (peritoneum), causing peritonitis. Infections of the uterus, fallopian tubes, ovaries, and/or peritoneum are called pelvic inflammatory disease Pelvic Inflammatory Disease (PID) Pelvic inflammatory disease is an infection of the upper female reproductive organs (the cervix, uterus, fallopian tubes, and ovaries). Pelvic inflammatory disease is usually transmitted during... read more Pelvic Inflammatory Disease (PID) .

Pathway From the Vagina to the Ovaries

In women, some organisms can enter the vagina and infect other reproductive organs. From the vagina, these organisms can enter the cervix and uterus and may reach the fallopian tubes and sometimes the ovaries.

Internal Female Genital Organs

In men, organisms that enter through the penis may infect the tube that carries urine from the bladder through the penis (urethra). Complications are uncommon if infections are treated quickly, but chronic infection of the urethra can cause the following:

  • Tightening of the foreskin, so that it cannot be pulled over the head of the penis

  • Narrowing of the urethra, blocking the flow of urine

  • Development of an abnormal channel (fistula) between the urethra and the skin of the penis

Occasionally in men, organisms spread up the urethra and travel through the tube that carries sperm from the testis (ejaculatory duct and vas deferens) to infect the epididymis (the coiled tube on top of each testis—see figure Pathway From the Penis to the Epididymis Pathway From the Penis to the Epididymis Pathway From the Penis to the Epididymis ).

Pathway From the Penis to the Epididymis

Occasionally in men, organisms spread up the urethra and travel through the tube that carries sperm from the testis (vas deferens) to infect the epididymis at the top of a testis.

Male Reproductive Organs

In both sexes, some STIs can cause persistent swelling of the genital tissues or infection of the rectum (proctitis).

Diagnosis of STIs

  • A doctor's evaluation

  • Examination of a sample of blood, urine, or discharge

Doctors often suspect an STI based on symptoms.

For many STIs, tests to identify the cause are limited or unavailable. Thus, doctors sometimes do not do tests to identify the cause. Instead, they choose treatment based on which organisms are most likely to cause the person's symptoms. Also, doctors may prefer to treat people at their first visit, before test results become available (which usually takes several days).

To identify the organism involved and thus confirm the diagnosis, doctors may take a sample of blood, urine, or discharge from the vagina or penis and examine it. The sample may be sent to a laboratory for the organisms to be grown (cultured) to aid in identification.

Some tests for STIs are designed to identify the organism’s unique genetic material (DNA or RNA). Sometimes techniques that increase the amount of the bacteria's genetic material are used. These tests are called nucleic acid amplification tests (NAATS). Because these techniques make the organisms easier to detect, urine samples can be used. Other tests check for the presence of antibodies that are produced by the immune system in response to the specific organism that is causing the infection. Doctors choose the type of test based on the STI suspected.

Screening for STIs

  • The disease being screened for is relatively common

  • People have a higher than average risk of having a disease (such as people with many sexual partners), or in whom a disease is particularly dangerous (such as pregnant women)

  • The screening test is easy and relatively cheap

  • There is effective treatment for the disease

Prevention of STIs

The following can help prevent STIs:

Condoms must be used correctly to be effective. Condoms should be applied before penetration. Correct use involves the following:

  • Use a new condom for each act of sexual intercourse.

  • Use the correct size condom.

  • Carefully handle the condom to avoid damaging it with fingernails, teeth, or other sharp objects.

  • Put the condom on after the penis is erect and before any genital contact with the partner.

  • Determine which way the condom is rolled by placing it on the index finger and gently trying to unroll it, but only a little bit. If it resists, turn it over, and try the other way. Then reroll it.

  • Place the rolled condom over the tip of the erect penis.

  • Leave 1/2 inch at the tip of the condom to collect semen.

  • With one hand, squeeze trapped air out of the tip of the condom.

  • If uncircumcised, pull the foreskin back before unrolling the condom.

  • With the other hand, roll the condom over the penis to its base and smooth out any air bubbles.

  • Make sure that lubrication is adequate during intercourse.

  • With latex condoms, use only water-based lubricants. Oil-based lubricants (such as petroleum jelly, shortening, mineral oil, massage oils, body lotions, and cooking oil) can weaken latex and cause the condom to break.

  • Hold the condom firmly against the base of the penis during withdrawal, and withdraw the penis while it is still erect to prevent slippage.

Not having sex (anal, vaginal, or oral) is the most reliable way to prevent STIs but is often unrealistic.

Treatment of STIs

  • Antibiotics or antiviral drugs depending on the STI

  • Simultaneous treatment of sex partners

Most STIs can be effectively treated with drugs (antibiotics for bacterial infections and antiviral drugs for viral infections). However, some new strains of bacteria and viruses have become resistant to some drugs, making treatment more difficult. Resistance to drugs Antibiotic resistance Bacteria are microscopic, single-celled organisms. They are among the earliest known life forms on earth. There are thousands of different kinds of bacteria, and they live in every conceivable... read more is likely to increase because drugs are sometimes misused.

People 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. Thus, sex partners should be tested and treated simultaneously.

More Information about STIs

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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