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 A, B, and C and infections of the digestive tract (which cause diarrhea), such as Salmonella infections, Campylobacter infections, shigellosis, giardiasis, amebiasis, and mpox (formally known as monkeypox).
Transmission
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
Kissing or close body contact—for pubic lice infestation, scabies, molluscum contagiosum, , and mpox.
From mother to child during pregnancy or during birth—for syphilis, herpes, chlamydia, gonorrhea, human immunodeficiency virus (HIV) infection, and human papillomavirus (HPV) infection
Breastfeeding—for HIV infection
Contaminated medical instruments—for HIV infection
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.
Image courtesy of Dr. Pirozzi via the Public Health Image Library of the Centers for Disease Control and Prevention.
Some of the effects of STIs increase the risk of getting other infections (such as HIV infection). For example, having irritated skin (inflammation, as occurs in gonorrhea or chlamydia) or sores (as occurs in herpes, syphilis, or chancroid) makes it easier for other infectious organisms to enter the body.
Complications
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
Cardiovascular (heart and blood vessel) and brain infections due to syphilis
Severe infections and rare cancers due to HIV
Cervical, vulvar, vaginal, anal, and throat cancer due to HPV
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) 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.
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. |
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. |
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).
If a person has one STI, such as gonorrhea, doctors also do tests for other STIs, such as chlamydia, syphilis, and HIV infection. Doctors do these other tests because people who have one STI have a relatively high chance of having another one.
Screening for STIs
Screening refers to doing tests for a disease in people who do not have symptoms. Screening is best when done when
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
Doctors recommend screening for STIs in people who are at increased risk for infection with chlamydia, gonorrhea, syphilis, and/or HIV. All sexually active women under age 25 or those older than 25 who are at high risk of infection should be screened for chlamydia annually, and all pregnant women should be screened for all 4 of these STIs.
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 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.
Viral STIs, especially genital herpes and HIV infection, usually persist for life. Antiviral medications can control but not yet cure these infections.
Prevention of STIs
The following can help prevent STIs:
Safer sex practices, including using a condom every time for oral, anal, or genital sex
Decreased risk of exposure to STIs by reducing the number of sex partners, not having high-risk sex partners (people with many sex partners or who do not practice safer sex), or practicing mutual monogamy or abstinence
Vaccination, available for some STIs
Circumcision (which can also reduce the spread of HIV to men)
Prompt diagnosis and treatment of STIs (to prevent spread to other people)
Identification of the sexual contacts of infected people, followed by counseling or treatment of these contacts
Vaccines are available for HPV infection, hepatitis A, and hepatitis B.
People at high risk of HIV infection may take medications prior to being exposed to prevent becoming infected (see HIV: Preventive treatment before exposure).
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.
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.
