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Overview of Sexually Transmitted Infections (STIs)


Sheldon R. Morris

, MD, MPH, University of California San Diego

Reviewed/Revised Feb 2023
Topic Resources

Sexually transmitted infection (STI) refers to an infection that is passed 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 disease that has developed from an STI.

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

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

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

  • Using condoms during genital sex can help prevent passing these infections from one person to another.

Sexual contact, including oral, anal, or genital sex, provides an opportunity for organisms to spread (be transmitted) from one person to another because it involves transfer of body fluids. Some infections that are spread through sexual contact can also be spread through kissing or close body contact.

STIs are relatively common. In the United States, over 25 million new cases of STIs occur each year; about half of the new cases occur in people ages 15 to 24 years (see also Centers for Disease Control and Prevention [CDC]: Sexually Transmitted Disease Surveillance 2020).

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

  • Unprotected sexual activity with one or more partners

  • Lack of education about safer sex practices

  • Reluctance to talk about safer sex practices with a partner

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

  • Lack of access to health care

  • Infections that do not cause symptoms, so people do not know they need to be tested or treated

  • The need to treat both sex partners simultaneously to avoid transmitting the infection between partners again

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

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 contact are usually 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 , 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 , and mpox (formally known as monkeypox) Mpox (Monkeypox) Mpox is caused by the monkeypox virus, which is related to the smallpox virus and causes a similar, but usually milder, illness. Mpox is caused by the monkeypox virus, which is related to the... read more Mpox (Monkeypox) .


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, or urination may be painful.

Some of the effects of STIs 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 is treated with antiretroviral medications. If untreated, it can cause... 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, or throat, or the membranes that cover... read more Gonorrhea or chlamydia Chlamydia 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 Chlamydia 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. It can occur in three stages of symptoms, separated by periods of apparent good health. Syphilis... read more Syphilis , or chancroid Chancroid Chancroid is a sexually transmitted infection caused by the bacteria Haemophilus ducreyi, which causes painful genital sores. Chancroid is a sexually transmitted infection (STI) that... 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. Damage to the fallopian tubes can result in infertility or a higher risk of a mislocated (ectopic Ectopic Pregnancy Ectopic pregnancy is attachment (implantation) of a fertilized egg in an abnormal location, such as a fallopian tube. In an ectopic pregnancy, the fetus cannot survive. Women with an ectopic... 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 often caused by a sexually... 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).

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 men and women, some STIs can cause persistent swelling of the genital tissues or infection of the urethra or 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 or a history of sexual contact with an infected partner.

To identify the organism involved and thus confirm the diagnosis, doctors may take a sample of blood, urine, or discharge from the vagina, cervix, or penis and examine it. The sample is usually sent to a laboratory for the organisms to be detected and identified; some tests for STIs can be done in the clinic.

Some tests for STIs are designed to identify the organism’s unique genetic material (DNA or RNA). 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 most likely infection(s).

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 more than one sex partner), or in whom a disease is particularly dangerous (such as pregnant women)

  • The screening test is easy and relatively inexpensive

  • There is effective treatment for the disease

Treatment of STIs

  • Antibiotics or antiviral medications depending on the STI

  • Treatment of complications, if present

  • If possible, simultaneous treatment of sex partners

Most STIs can be effectively treated with medications (antibiotics for bacterial infections and antiviral medications for viral infections). However, some new strains of bacteria and viruses have become resistant to some medications, making treatment more difficult. Resistance to medications 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 medications 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.

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.

More Information

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

Drugs Mentioned In This Article

Generic Name Select Brand Names
Fleet, Kondremul, Liqui-Doss, Muri-Lube
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