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Overview of Sexually Transmitted Diseases (STDs)


Sheldon R. Morris

, MD, MPH, University of California San Diego

Last full review/revision Jan 2021| Content last modified Jan 2021
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Topic Resources

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

  • Sexually transmitted diseases 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 diseases 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 diseases (STDs), also called sexually transmitted infections (STIs), are relatively common. For example, an estimated 20 million new cases of STDs 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 STDs.

Several factors make prevention of STDs 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 STDs

  • 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 STDs to be rapidly spread worldwide

Causes of STDs


Types of Sexually Transmitted Diseases





Genital warts (caused by the human papillomavirus)

Molluscum contagiosum (caused by a poxvirus)

Parasitic (protozoan)


Scabies (due to burrowing mites)

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 STDs. 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, and amebiasis.


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

Symptoms of STDs

Symptoms of STDs 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). For example, having irritated skin (inflammation, as occurs in gonorrhea or chlamydial infection) or sores (as occurs in herpes, syphilis, or chancroid) makes it easier for other infectious organisms to enter the body.


When STDs 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

  • AIDS due to HIV

  • Cervical, rectal, 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 (see figure Pathway From the Vagina to the Ovaries). Damage to the uterus and fallopian tubes can result in infertility or 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.

Pathway From the Vagina to 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—see figure 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.

Pathway From the Penis to the Epididymis

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

Diagnosis of STDs

  • A doctor's evaluation

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

Doctors often suspect an STD based on symptoms.

For many STDs, 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 STDs 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 STD suspected.

If a person has one STD, such as gonorrhea, doctors also do tests for other STDs, such as chlamydial infection, syphilis, and HIV infection. Doctors do these other tests because people who have one STD have a relatively high chance of having another one.

Screening for STDs

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 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

Doctors recommend screening for STDs in people who are at increased for chlamydia, gonorrhea, and syphilis. All adult women under age 25 should be screened for chlamydia annually, and all pregnant women should be screened for all three of these STDs.

Prevention of STDs

The following can help prevent STDs:

  • Avoidance of unsafe sex practices, such as frequently changing sex partners or having sexual intercourse with prostitutes or with partners who have other sex partners

  • Circumcision (which can also reduce the spread of HIV from women to men)

  • Prompt diagnosis and treatment of STDs (to prevent spread to other people)

  • Identification of the sexual contacts of infected people, followed by counseling or treatment of these contacts

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 STDs but is often unrealistic.

The only vaccines available are those for HPV infection, hepatitis A, and hepatitis B.

Treatment of STDs

  • Antibiotics or antiviral drugs depending on the STD

  • Simultaneous treatment of sex partners

Most STDs 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 is likely to increase because drugs are sometimes misused.

People who are being treated for a bacterial STD 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 STDs, especially genital herpes and HIV infection, usually persist for life. Antiviral drugs can control but not yet cure these infections.

More Information about STDs

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

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
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