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What causes a sexually transmitted disease or sexually transmitted infection (STD/STI)?

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There are two major causes of STDs/STIs:

  • Bacteria, including chlamydia, gonorrhea, and syphilis
  • Viruses, including HIV/AIDS, herpes simplex virus, human papillomavirus (HPV), hepatitis B virus, and cytomegalovirus (CMV; pronounced sahy-toh-MEG-uh-loh-vahy-ruhs), yeasts and protozoan parasites, such as Trichomonas vaginalis (pronounced TRIK-uh-MOH-nuss vaj-uh-NAHY-lis), or insects such as crab lice or scabies mites, cause STDs/STIs.1

Any STD/STI can be spread through sexual intercourse, and some STDs/STIs also are spread through oral sex and sex play. Ejaculation does not have to occur for an STD/STI to be passed from person to person. Sharing contaminated needles used to inject drugs or using contaminated body piercing and tattooing equipment also can transmit some infections, such as HIV or hepatitis B and C.

A few diseases (such as CMV and molluscum contagiosum) can be sexually transmitted but are also spread through nonsexual, close contact. Regardless of how a person is exposed, once a person is infected by an STD/STI, he or she can spread the infection to other people through oral, vaginal, or anal sex, even if he or she has no symptoms.

STDs/STIs are of particular concern in pregnant women, because some infections can be passed on to the infant before birth or during delivery. However, the risk of transmission from mother to infant can be lowered, and it is important for every expectant mother to be screened.

For example, HIV can be passed from mother to infant during pregnancy before birth, at the time of delivery, or after birth during breastfeeding.2 This transmission can be prevented through treatment with certain medications during pregnancy and near delivery. After birth, women who have HIV should refrain from breastfeeding their infants if safe alternatives, such as infant formula, are available, further reducing the infant's risk.3

In other cases, if the mother has an infection such as gonorrhea or herpes, in which risks of transmission are high during delivery, other steps can be taken to reduce the likelihood that the infant will be infected. In these instances, health care providers can treat the pregnant woman for the STD/STI before birth, or the infant can be delivered by cesarean section (also referred to as C section).3

CMV affects about 1% of all births. A pregnant woman infected with CMV can transmit the infection to the fetus in the womb, or it can be passed to the infant during delivery or by breastfeeding. She could also pass it to her newborn after birth if the child comes into contact with her body fluids (for example, saliva or urine) carrying the virus. If a health care provider suspects that a woman has a CMV infection during pregnancy, an ultrasound examination, blood tests, and other tests are done to assess the health of the fetus. Most infants who were infected with the virus during pregnancy do not have any detectable problems after birth. But, 10% or 20% will have serious problems, including deafness and intellectual disabilities. If fetal testing shows increased risk of serious problems, some women opt to end the pregnancy. Researchers are studying antiviral drugs, immune treatments, and other medical approaches to control the infection during pregnancy. Some research is focusing on vaccines to prevent CMV infection.4


  1. Illinois Department of Public Health. Facts about vaginitis and vaginal infections. Retrieved June 27, 2012, from http://www.idph.state.il.us/about/womenshealth/factsheets/vag.htm External Web Site Policy [top]
  2. Medline Plus. (2011, September 12). Genital Herpes. Retrieved July 27, 2012, from http://www.nlm.nih.gov/medlineplus/ency/article/000857.htm [top]
  3. The American College of Obstetricians and Gynecologists. (2011, May) Frequently asked questions. HIV and pregnancy. Retrieved June 27, 2012, from http://www.acog.org/~/media/For%20Patients/faq113.pdf?dmc=1&ts=20120711T1503446877 External Web Site Policy [top]
  4. Carlson, A., Norwitz, E. R., & Stiller, R. J. (2010). Cytomegalovirus infection in pregnancy: should all women be screened? Reviews in Obstetrics and Gynecology, 3(4),172-179. PMID: 21364849 [top]

Last Updated Date: 04/02/2013
Last Reviewed Date: 05/28/2013
Vision National Institutes of Health Home BOND National Institues of Health Home Home Storz Lab: Section on Environmental Gene Regulation Home Machner Lab: Unit on Microbial Pathogenesis Home Division of Intramural Population Health Research Home Bonifacino Lab: Section on Intracellular Protein Trafficking Home Lilly Lab: Section on Gamete Development Home Lippincott-Schwartz Lab: Section on Organelle Biology