Sexually Transmitted Infections (STIs): Overview

Sexually transmitted infections (STIs) are infections that pass from one person to another through sexual contact, which includes oral, genital, or anal sex.

What Are Sexually Transmitted Infections?

STIs can be caused by bacteria, viruses, or parasites. Although the symptoms of a particular STI depend on the specific infection, many STIs cause vaginitis (vah-jih-NYE-tis), an inflammation of the vagina * often accompanied by an abnormal discharge (fluid released from the body), and urethritis (yoo-ree-THRY-tis), an inflammation of the urethra (the tube through which urine passes from the bladder * to the outside of the body), which can make urination painful. Several STIs can produce blisters or sores on the penis * , vagina, rectum * * , a condition called cervicitis (sir-vih-SYEtis), or to the uterus * and fallopian tubes * , a condition known as pelvic inflammatory disease (PID). In men, STIs may spread to the testicle (causing epididymitis * ) or prostate * (causing prostatitis, which is inflammation of the prostate). Sometimes, a person may contract several STIs. People who have an STI are at increased risk of infection with human immunodeficiency virus (HIV * ), the virus that causes acquired immunodeficiency syndrome (AIDS * ).

How Common Are Sexually Transmitted Infections?

STIs are common in the United States, and medical professionals diagnose 20 million new cases every year. About half of these new infections occur in young people between the ages of 15 and 24. Generally, STI incidence declined in the United States from 2000 through 2015, although rates of chlamydia, syphilis, and gonorrhea went up between 2013 and 2015. In addition, rates of STIs in men who have sex with men have increased. Improvements in STI screening and testing are believed to be responsible for the increase in reported STIs in the early 21st century.

Are STIs Contagious?

STIs are contagious and are transmitted through sexual contact that involves vaginal, anal, or oral sex. The diseases can spread between people of the opposite sex or people of the same sex. The germs that cause many STIs move from person to person through semen (the sperm-containing whitish fluid produced by men), vaginal (VAH-jih-nul) fluids, or blood. Other STIs, such as herpes and genital warts, can spread by intimate skin-to-skin contact, often with the sores that the disease causes. A mother can transmit certain STIs to her baby during pregnancy or childbirth. STIs do not pass from one person to another by simply hugging, shaking hands, or sharing utensils.

What Are Some Common Sexually Transmitted Infections?


The bacterium Neisseria gonorrhoeae (nye-SEER-e-uh gah-no-REE-eye), also called the gonococcus, causes gonorrhea (gahnuh-REE-uh). Women who are infected may not have any symptoms. When they do, symptoms include vaginal discharge and pain during urination. Lower belly pain usually occurs when the infection has spread past the cervix and causes pelvic inflammatory disease (PID). Most men with gonorrhea have a discharge from the penis and pain during urination. Doctors prescribe antibiotics * to patients with gonorrhea. People who have gonorrhea are often also infected with chlamydia.

Syphilis * , muscle aches, and headaches. If the disease goes untreated, it can progress to the third and most serious stage, when it may damage the bones, organs, and nervous system. At this point, it can result in blindness, paralysis * , dementia * , heart problems, and sometimes even death. Doctors can treat syphilis with antibiotics. About 20,000 cases of syphilis are reported in the United States each year.

Skin abscesses caused by gonococcal infection.

Skin abscesses caused by gonococcal infection.
Biophoto Associates/Science Source.
Herpes simplex virus

Herpes simplex (HER-peez SIM-plex) virus causes herpes. Herpes comes in two types, one of which—type 2—usually spreads through sexual contact and causes genital herpes. In a person with genital herpes, small painful blisters develop on the vagina, cervix, penis, buttocks, or thighs. Once infection occurs, the herpesvirus remains in the body and can cause blisters throughout a person's life. Antiviral medications may shorten outbreaks of symptoms and make them less severe, but they do not kill the virus. In the United States, about one out of every six people aged 14 to 49 years has genital herpes.


Chlamydia (kla-MIH-dee-uh) results from infection with the bacterium Chlamydia trachomatis (kla-MIH-dee-uh truh-KO-mahtis). In many infected people, it produces no symptoms. The most common symptoms in both men and women are discharge and pain during urination. Because infection with chlamydia often goes unnoticed, it can spread and produce other symptoms, including epididymitis in men and PID in women. The Centers for Disease Control and Prevention estimates that chlamydia bacteria infect 1.5 million people in the United States each year. Medical professionals use antibiotics to treat the infection effectively.


Infection with HIV damages immune system cells in the body that normally fight infections, leaving the body unable to defend itself against a variety of illnesses. A person can be infected with HIV and not have AIDS, but most people with untreated HIV infection eventually develop AIDS. The first symptoms of HIV infection are fever, muscle aches, sore throat, and in some cases, a rash that looks somewhat like that of measles * . Other symptoms usually take much longer—perhaps years—to appear and may include rapid weight loss, recurring fever, a dry cough, night sweats, pneumonia * , white spots on the tongue or throat, longlasting diarrhea, and skin rashes and yeast infections. A person with AIDS may also have memory loss, depression, and extreme tiredness.

Antibiotics used to treat STIs

Antibiotics used to treat STIs
Table by PreMediaGlobal. © 2016 Cengage Learning®.
Human papillomavirus

Genital and anal warts are caused by human papillomavirus (pah-pih-LO-mah-vy-rus), or HPV, a common virus. The warts are soft and skin-colored, and they can grow alone or in bunches on the genitals; on the skin around the genitals, rectum, or buttocks; or in the vagina or cervix. Like herpes, genital warts can reappear many times because once this type of virus enters the body, it remains there for life. Doctors can remove genital warts by freezing, burning, or cutting them off, or coating them with medication that destroys the warts. In women, infection with HPV can affect the cells of the cervix, which may lead to cervical cancer.


A parasite causes trichomoniasis (trih-ko-mo-NYEuh-sis), which is a common STI. It is considered the most common curable STI. As of 2016, an estimated 3.7 million people in the United States had the infection. Unfortunately only about 30 percent develop any symptoms of the infection, which means that it is more easily spread. Most women with trichomoniasis have a frothy yellow foul-smelling vaginal discharge, along with itching and irritation in the vagina and discomfort during sex and urination. Men with this STI typically do not have symptoms. Those who do have symptoms may feel irritation in the penis or a burning sensation after they urinate or ejaculate * . Most cases of trichomoniasis are treated with a single dose of the oral antibiotic metronidazole. Doctors recommend that people who are infected not have sex until they have completed treatment to limit the risk of spreading the infection. Treating all sexual partners of someone who has trichomoniasis, even if they have no symptoms, also is suggested as a way to prevent a new round of infection or the spread of the disease.

Can Sexually Transmitted Infections Be Prevented?

* are available against some STIs, such as specific HPVs. These vaccines are most effective when given before an individual has engaged in any sexual contact.

Many cases of chlamydia, gonorrhea, and syphilis continue to go undiagnosed and unreported; and data on several additional STIs, such as human papillomavirus, herpes simplex virus, and trichomoniasis, are not routinely reported to the CDC.

See also AIDS and HIV Infection • Bacterial Infections • Cervical Cancer • Chlamydial Infections • Epididymitis • Fungal Infections • Genital Warts • Gonorrhea • Herpes Simplex Virus Infections • Human Papillomavirus (HPV) • Nonspecific Urethritis • Pelvic Inflammatory Disease (PID) • Syphilis • Urinary Tract Infections • Viral Infections


Books and Articles

Jones, Estelle. Sexually Transmitted Infections. New Brunswick, NJ: ML Books International, 2015


Centers for Disease Control and Prevention. “Chlamydia – CDC Fact Sheet.” (accessed July 8, 2016).

Centers for Disease Control and Prevention. “Genital Herpes.” CDC. gov. (accessed July 8, 2016).

Centers for Disease Control and Prevention. “Gonorrhea – CDC Fact Sheet.” . (accessed July 8, 2016).

Centers for Disease Control and Prevention. “Human Papillomavirus (HPV).” . (accessed July 8, 2016).

Centers for Disease Control and Prevention. “Syphilis – CDC Fact Sheet.” . (accessed July 8 2016).

Centers for Disease Control and Prevention. “Trichomoniasis.” . (accessed July 8, 2016).


American Sexually Transmitted Diseases Association. 1005 Slater Rd., Suite 330, Durham, NC 27709. Telephone: 919-861-9399. Website: (accessed July 8, 2016).

Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-232-4636. Website: (accessed July 8, 2016).

Food and Drug Administration. 10903 New Hampshire Ave., Silver Spring, MD 20993. Toll-free: 888-463-6332. Website: (accessed July 8, 2016).

National Coalition of STD Directors (NCSD). 1029 Vermont Ave. NW, Suite 500, Washington, DC 20005. Telephone: 202-842-4660. Website: (accessed July 8, 2016).

National Institute of Allergy and Infectious Diseases. 5601 Fishers Ln., MSC 9806, Bethesda, MD 20892-9806. Toll-free: 866-284-4107. Website: (accessed July 8, 2016).


* bladder (BLAD-er) is the structure that stores urine produced by the kidneys before discharge from the body.

* penis (PEE-nis) refers to the male genital organ that carries the duct for the transfer of sperm during sexual intercourse and for eliminating urine. It consists largely of erectile tissue.


* cervix (SIR-viks) is the lower narrow end of the uterus that opens into the vagina.

* uterus (YOO-teh-rus) is the muscular pear-shaped internal organ in a woman in which a baby develops until birth.

* fallopian tubes (fa-LO-pee-an) are the two slender tubes that connect the ovaries and the uterus in women. They carry the ova, or eggs, from the ovaries to the uterus.

* epididymitis (eh-pih-dih-duh-MY-tis) is a painful inflammation of the epididymis, a structure attached to the testicles.

* prostate (PRAH-state) is a male reproductive gland located near the point at which the bladder joins the urethra. The prostate produces the fluid part of semen.

* HIV, or human immunodeficiency (HYOO-mun ih-myoono-dih-FIH-shen-see) virus, is the virus that causes AIDS (acquired immunodeficiency syndrome).

* AIDS, or acquired immunodeficiency (ih-myoo-no-dih-FIH-shensee) syndrome, is an infection that severely weakens the immune system. It is caused by the human immunodeficiency virus (HIV).

* antibiotic (an-tie-by-AH-tik) a drug that kills or slows the growth of bacteria.

* lymph nodes (LIMF nodz) are small bean-shaped masses of tissue containing immune system cells that fight harmful microorganisms. Lymph nodes may swell during infections.


* dementia (dih-MEN-sha) is a gradually worsening loss of mental abilities, especially memory.

* measles (ME-zuls) is a viral respiratory infection that is best known for the rash of large flat red blotches that appear on the arms, face, neck, and body.

* pneumonia (nu-MO-nyah) is inflammation of the lungs, usually caused by viruses or bacteria.

* ejaculate (e-JAH-kyoo-late) means to discharge semen from the penis.

