Skip to main content Skip to Search Box

Definition: sexually transmitted disease (STD) from Philip's Encyclopedia

Any disease that is transmitted by sexual activity. It encompasses a range of conditions that are spread primarily by sexual contact, although some may also be transmitted in other ways also. These include Acquired Immune Deficiency Syndrome (AIDS), syphilis, gonorrhoea, chlamydia, cervical cancer and viral hepatitis. STDs are a significant public health problem.

Summary Article: Sexually Transmitted Diseases
From Encyclopedia of Global Health

Sexually transmitted diseases, often also referred to as sexually transmitted infections or venereal diseases, are infectious diseases that are primarily transmitted via vaginal, anal, or oral sex, or via other sexual contact. Pathogens infect via the mucous membranes of the penis, vagina, or mouth, which are generally more porous than skin. Genital fluids and blood generally have greater pathogen content than saliva. Sexually transmitted diseases include genital herpes, the human papilloma virus (HPV), gonorrhea, chlamydia, syphilis, and the human immunodeficiency virus (HIV). While other pathogens, such as meningitis and hepatitis B, can be transmitted via sexual contact, these infections are not classified as sexually transmitted infections as sexual contact is not the primary route of transmission.

Similarly, sexually transmitted pathogens can also often be transmitted via other routes that permit contact with infectious material, such as sharing needles that have contacted contaminated blood. Herpes and HPV can also be transmitted via skin-to-skin contact. Many sexually transmitted infections, including syphilis, gonorrhea, genital herpes, and HIV, can also be transmitted from mothers to infants during child-birth and breast-feeding.


Sexually transmitted diseases are caused by a variety of bacterial and viral pathogens, and those infected can present with a range of symptoms. One of the most common sexually transmitted infections, chlamydia, is caused by the Chlamydia trachomatis bacteria and can cause painful urination, fever, unusual discharge or bleeding from the genitals, and abdominal pain. Chlamydia is often referred to as a “silent infection,” however, as up to half of males and three-quarters of females who are infected do not exhibit symptoms and do not know they are infected. Among women who are asymptomatic, approximately half who are not treated will developed pelvic inflammatory disorder (PID), which can cause scarring in the fallopian tubes, uterus, and ovaries, and can cause serious complications including chronic pelvic pain and complications in pregnancy.

Gonorrhea, another bacterial sexually transmitted infection, is caused by the Neisseria gonorrhoeae bacteria and can also result in pelvic inflammatory disorder, as well as infections of the urethra, cervix, uterus, epididymis, and testicles. Symptoms include painful urination and intercourse, discharge or bleeding from the genitals, cramps, vomiting, fever, and scrotal swelling. As with chlamydia, up to two-thirds of those infected with gonorrhea may be asymptomatic. These asymptomatic infections, which can lead to other reproductive health problems, are an important reason why sexually active people should be routinely screened for sexually transmitted pathogens, even when they exhibit no symptoms.

Syphilis is also a bacterial sexually transmitted disease, caused by the Treponema pallidum bacteria. Primary syphilis occurs 10 to 90 days following infection, and patients experience a single sore near the site of infection. Secondary syphilis occurs one to six months following infection, and is characterized by a skin rash on the trunk and extremities (including the palms of hands and soles of feet), fever, sore throat, headache, weight loss, and enlarged lymph nodes. Tertiary syphilis occurs 1 to 50 years following secondary syphilis, and is characterized by tumor-like growths on the skin, mucous membranes, skeleton, and elsewhere. If left untreated, tertiary syphilis can cause damage to the heart, brain, and nervous system, and can be fatal. Syphilis gained notoriety in the United States in the course of one of the most unethical medical experiments involving human subjects, the Tuskegee Syphilis Study, during which infected African-American men were left untreated in order to study the natural course of the disease, even though effective antibiotics had long been available.

Viral sexually transmitted diseases include genital herpes, HPV, and HIV. There are more than 100 different strains of HPV, approximately 30 of which are commonly transmitted sexually, and which infect mucous membranes and skin, and can cause lesions also known as genital warts, although many of those infected exhibit no visible lesions. Certain strains of HPV, especially HPV 16 and 18, can cause cancer of the cervix, although a vaccine against these high-risk strains is now available. A screening test, known as the Pap smear, is also highly effective at identifying cancerous lesions, although mortality from cervical cancer is still high worldwide among women without access to Pap smears and treatment.


Genital herpes is caused by the herpes simplex virus, which causes painful sores on skin and mucous membranes. The herpes simplex virus is also responsible for cold sores on or near the lips. Outbreaks of sores are generally preceded by itching, tingling, or irritation. Following infection, the virus lies dormant in the cell bodies of neurons, growing toward the skin with the axon, and then falling back into the cell body following an outbreak. This disease cannot be cured, although antiviral medications can reduce the severity and length of active outbreaks. Herpes simplex infections acquired during childbirth in infants can cause brain damage, and thus pregnant women are often actively screened and treated in order to prevent transmission.


Perhaps the most deadly of sexually transmitted infections, HIV is the pathogen ultimately responsible for causing the acquired immunodeficiency syndrome (AIDS), which causes the immune system to fail and usually results in death from secondary infections. While antiretroviral medications can reduce viral load and the clinical progression to AIDS, there is no cure or vaccine against HIV. Most infected individuals exhibit primary flu-like symptoms and a rash approximately one to six weeks following infection. Following a latency period of between one and 20 years, most infected individuals ultimately go on to exhibit symptoms of AIDS, including lowered T-cell counts, weight loss, and vulnerability to secondary infections such as pneumonia, oral infections, Kaposi’s sarcoma, and toxoplasmosis. Some individuals have been infected with HIV for several years but have not yet developed symptoms of AIDS. While HIV is not solely transmitted via sexual intercourse, sexual transmission of HIV is the primary means of viral transmission, especially in countries with high prevalence of infection, including most of sub-Saharan Africa.

Sexually transmitted diseases are also caused by protozoa and parasites. Pubic lice Phthiris pubis, colloquially known as crabs, are parasitic lice that live on human hair, especially pubic hair, and are primarily transmitted via sexual contact. They can cause intense itching or rash.

These parasites can also be transmitted via sharing infected clothing or bedding. Adult pubic lice can be killed using medicated topical shampoos, although pubic hair must also be removed or combed to remove eggs (nits) that can cause further outbreaks after hatching. Sheets or clothes worn by infected persons must also be sterilized or destroyed. The scabies mite Sarcoptes scabei can also be transmitted sexually, which can also cause intense itching and an allergic response.

The single-celled protozoan Trichomonas vaginalis is also spread via sexual contact, usually infecting the urethra in men and the vagina in women. Symptoms generally occur five to 28 days following infection and can include vaginal discharge, painful urination, inflammation of the genital region, and abdominal pain. However, most infections in men and approximately 20 percent of those in women are asymptomatic. Antiprotozoal drugs can be used to cure the disease.

Although not caused by the transmission of a specific pathogen, bacterial vaginosis (BV), a common reproductive tract infection among women of reproductive age, is also associated with sexual behavior. When the normal bacterial balance of the vagina is disrupted, often by sexual intercourse or through use of certain contraceptive methods (especially the in intrauterine device [IUD]), vaginosis can occur, leading to itching, pain, odor, or vaginal discharge. While BV often causes no harmful long-term symptoms, these infections can increase the transmission rates of sexually transmitted infections, including HIV, chlamydia, and gonorrhea, and is also associated with pelvic inflammatory disease and pregnancy complications, including delivering babies of lowered birth weight. Although symptoms often clear up spontaneously, women with BV can be treated effectively with common antibiotics.


There are several epidemiological factors that help determine the prevalence of sexually transmitted diseases in a population. Bacterial infections, including chlamydia and gonorrhea, can be cured using antibiotics, but those infected do not always have access to antibiotics and physicians, nor do asymptomatic patients always realize that they are infected. Anti-biotic-resistant strains of bacteria are also becoming more prevalent, especially when antibiotics are incorrectly prescribed or ingested. Transmission rates of most sexually transmitted infections are enhanced when individuals are coinfected with another sexually transmitted disease, especially when these individuals have open sores in the vaginal canal, on the penis, or on the mucous membranes of the mouth.

Individuals are often more infectious at certain points in the course of disease than at others: for instance, those infected with genital herpes are only infectious when experiencing an active outbreak characterized by genital ulcers, while those infected with HIV are generally most infectious shortly following initial infection, as well as when the disease has progressed to AIDS. For some diseases, including HIV, receptive vaginal or anal sex is associated with higher rates of disease transmission as the receptive partner is more likely to suffer a tear in a mucous membrane than the penetrative partner, allowing a pathogen to pass through the mucous membrane more easily. Male-to-female transmission is thus often much more physiologically efficient than female-to-male transmission.

Social factors also determine the prevalence and incidence levels of sexually transmitted infections in a population. Sexually transmitted infections are concentrated in young people, with up to 60 percent of all new infections occurring in those under the age of 25. Properties of sexual networks, defined as the patterns of sexual mixing between individuals, often influence transmission routes. Sex workers and other highly sexually active persons are often instrumental in passing pathogens between otherwise disparate segments of populations, and thus are often the targets of public health campaigns. Long-distance travel has helped spread infections far beyond local areas, and infection outbreaks often spread along migration and transportation routes. Gender inequality, marriages between adolescent women and older men, and sexual violence has been associated with increased transmission of HIV and other sexually transmitted infections in many population-based studies.

Prevention of sexually transmitted diseases is often accomplished via public health campaigns that encourage “safer sex” practices, such as using male or female condoms, reducing overall numbers of sexual partners, remaining faithful in a mutually monogamous relationship, delaying sexual debut, and remaining abstinent. Other campaigns encourage individuals to get tested frequently for disease status, especially when beginning a new sexual relationship. Some controversy surrounds testing for incurable sexually transmitted diseases, such as HIV, where the only benefit to those getting tested is their ability to avoid passing on the disease to others. This controversy has lessened in recent years as new pharmaceuticals for treating genital herpes outbreaks and HIV, among others, have become available, giving individuals a greater incentive to get tested. Contact tracing, whereby infected individuals inform all sexual contacts of their disease status, can also be effective in preventing localized disease outbreaks.

In many countries, including the United States, accurate scientific information regarding the various risks of specific sexual practices and preventative measures is often difficult to access, particularly in schools, based on politics and customs surrounding sexual mores and practices, particularly for women, unmarried adolescents, homosexuality, and same-gender sex acts. Unlike many medical illnesses, those infected are often blamed for their disease or for transmitting disease, due to the politics of sexuality, sexual behavior, and personal sexual freedom.

Despite associating infections with a few often vilified groups such as sex workers, highly sexually active individuals and homosexuals, sexually transmitted diseases are among the most common infections in a population and affect people of all sexual orientations, ages, races, and ethnic backgrounds. The social stigma surrounding sexually transmitted infections discourages open discussion and accurate information, thus individuals are often unaware of their risk of disease or are uninformed regarding the need for or means of protection and testing. Ignorance of disease risk is only enhanced by the fact that many sexually transmitted infections are asymptomatic, and thus individuals who only examine themselves or their sexual partners for visible signs or symptoms of disease, rather than getting tested and discussing the results with their partners, will underestimate their risk of infection.

Clinically, sexual health is often lumped together with reproductive health, with Pap smears for HPV detection in particular often being considered routine care for reproductive-age women. Detection and treatment of sexually transmitted diseases are, however, often hampered by unwillingness on the part of either patients or clinicians (or both) to discuss sexual behavior. For this and other reasons, many countries have initiated a system of sexually transmitted disease clinics apart from other primary healthcare facilities, where individuals can be tested and treated anonymously.

  • AIDS; Herpes Simplex; Pregnancy.

  • H. Hunter Handsfield, Color Atlas and Synopsis of Sexually Transmitted Diseases (McGraw-Hill Professional, 2000).
  • Anita L. Nelson; Jo Ann Woodward, eds., Sexually Transmitted Diseases: A Practical Guide for Primary Care (Current Clinical Practice) (Humana Press, 2007).
  • Lawrence R. Stanberry; David I. Bernstein, eds., Sexually Transmitted Diseases: Vaccines, Prevention and Control (Academic Press, 2000).
  • U.S. Department of Health And Human Serv, Sexually Transmitted Diseases - Updated 2006 (International Medical Publishing, 2007).
  • Annie Dude
    University of Chicago
    Copyright © 2008 by SAGE Publications, Inc.

    Related Articles

    Full text Article Sexually Transmitted Diseases
    Encyclopedia of Microbiology

    Abbreviations AIDS acquired immunodeficiency syndrome CSF cerebrospinal fluid DFA direct fluorescent...

    Full text Article Sexually Transmitted Infections
    Encyclopedia of Adolescence

    Introduction Adolescents comprise only 25% of the sexually active population, but acquire almost 50% of all sexually transmitted infections...

    Full text Article Sexually Transmitted Infections
    Global Social Issues: An Encyclopedia

    Sexually transmitted infections (STIs) are illnesses that are generally spread through sexual intercourse, including oral, anal, and vaginal sex. Th

    See more from Credo