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25.4: Sexually Transmitted Infections

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • Explain what causes STIs and how they can be prevented.
  • Identify and describe three common bacterial STIs.
  • Identify and describe three common viral STIs.


sexually transmitted bacterial infection that is the most common STI in the United States
genital herpes
sexually transmitted infection caused by a herpes virus that is characterized by periodic outbreaks of blisters on the genitals
genital warts
small, rough growths on the genitals caused by a sexually transmitted infection with human papillomavirus (HPV)
common sexually transmitted infection that is caused by bacteria
hepatitis B
inflammation of the liver caused by infection with hepatitis B virus, which is often transmitted through sexual contact
human papilloma virus (HPV)
sexually transmitted virus that causes genital warts and cervical cancer
sexually transmitted infection (STI)
infection caused by a pathogen that spreads mainly through sexual contact; also known as sexually transmitted disease (STD)
sexually transmitted infection caused by bacteria that may eventually be fatal if untreated
common sexually transmitted infection that is caused by protozoa


A shocking statistic made headlines in 2008. A recent study had found that one in four teen girls in the U.S. had a sexually transmitted infection. A sexually transmitted infection (STI) (also known as a sexually transmitted disease, or STD) is an infection caused by a pathogen that spreads mainly through sexual contact. Worldwide, a million people a day become infected with STIs. The majority of them are under the age of 25. For a video about sexually transmitted infections, go to this link: http://www.youtube.com/watch?v=Bazh6p5rOFM.

Understanding Sexually Transmitted Infections

To be considered an STI, an infection must have only a small chance of spreading naturally in ways other than sexual contact. Some infections that can spread through sexual contact, such as the common cold, spread more commonly by other means. These infections are not considered STIs.

Pathogens that Cause STIs

STIs may be caused by several different types of pathogens, including protozoa, insects, bacteria, and viruses. For example: Protozoa cause an STI called trichomoniasis. The pathogen infects the vagina in females and the urethra in males, causing symptoms such as burning and itching. Trichomoniasis is common in young people. Pubic lice, like the one in Figure below, are insect parasites that are transmitted sexually. They suck the blood of their host and irritate the skin in the pubic area.

Public Louse. Public lice like this one are only about as big as the head of a pin.

Most STIs are caused by bacteria or viruses. Several of them are described below. Bacterial STIs can be cured with antibiotics. Viral STIs cannot be cured. Once you are infected with a viral STI, you are likely to be infected for life.

How STIs Spread

Most of the pathogens that cause STIs enter the body through mucous membranes of the reproductive organs. All sexual behaviors that involve contact between mucous membranes put a person at risk for infection. This includes vaginal, anal, and oral sexual behaviors. Many STIs can also be transmitted through body fluids such as blood, semen, and breast milk. Therefore, behaviors such as sharing injection or tattoo needles is another way these STIs can spread. Why are STIs common in young people? One reason is that young people often take risks. They may think, “It can’t happen to me.” They also may not know how STIs are spread, so they don’t know how to protect themselves. In addition, young people may have multiple sexual partners.

Preventing STIs

The only completely effective way to prevent infection with STIs is to avoid sexual contact and other risky behaviors. Using condoms can lower the risk of becoming infected with STIs during some types of sexual activity. However, condoms are not foolproof. Pathogens may be present on areas of the body not covered by condoms. Condoms can also break or be used incorrectly.

Bacterial STIs

Many STIs are caused by bacteria. Some of the most common bacterial STIs are chlamydia, gonorrhea, and syphilis.


Chlamydia is the most common STI in the U.S. As shown in the graph in Figure below, females are much more likely than males to develop chlamydia. Like most STIs, rates of chlamydia are highest in teens and young adults.

This graph shows the number of cases of chlamydia per 100,000 people in the U.S. in 2004. Which age group had the highest rates? How much higher were the rates for females aged 15–19 than for males in the same age group?

Symptoms of chlamydia may include a burning sensation during urination and a discharge from the vagina or penis. Chlamydia can be cured with antibiotics, but often there are no symptoms, so people do not seek treatment. Untreated chlamydia can lead to more serious problems, such as pelvic inflammatory disease (PID). This is an infection of the uterus, fallopian tubes, and/or ovaries. It can scar a woman’s reproductive organs and make it difficult for her to become pregnant. To learn more about chlamydia, watch the video at this link: http://www.youtube.com/watch?v=qaazmU8YU7E&feature=fvw.


Gonorrhea is another common STI. Symptoms of gonorrhea may include painful urination and discharge from the vagina or penis. Gonorrhea usually can be cured with antibiotics, although the bacteria have developed resistance to many of the drugs. Gonorrhea infections may not cause symptoms, especially in females, so they often go untreated. Untreated gonorrhea can lead to PID in females. It can lead to inflammation of the reproductive organs in males as well.


Syphilis is less common than chlamydia or gonorrhea but more serious if untreated. Early symptoms of syphilis infection include a small sore on or near the genitals. The sore is painless and heals on its own, so it may go unnoticed. If treated early, most cases of syphilis can be cured with antibiotics. Untreated syphilis can cause serious damage to the heart, brain, and other organs. It may eventually lead to death.

Viral STIs

STIs caused by viruses include genital herpes, hepatitis B, genital warts, and HIV/AIDS, which is described in Chapter 24.

Genital Herpes

Genital herpes is an STI caused by a herpes virus. In the U.S., as many as one in four people are infected with the virus. Symptoms of genital herpes include painful blisters on the genitals (see Figure below). The blisters usually go away on their own, but the virus remains in the body, causing periodic outbreaks of blisters throughout life. Outbreaks may be triggered by stress, illness, or other factors. A person with genital herpes is most likely to transmit the virus during an outbreak. To learn more about genital herpes, watch the video at this link: http://www.youtube.com/watch?v=eXK6GKe1kOw&feature=PlayList&p=1386BB276DFC6837&playnext_from=PL.

Genital Herpes Blisters. Blisters like these on the genitals are a sign of genital herpes.

Hepatitis B

Hepatitis B is inflammation of the liver caused by infection with the hepatitis B virus. In many people, the immune system quickly eliminates the virus from the body. However, in a small percentage of people, the virus remains in the body and continues to cause illness. It may eventually damage the liver and increase the risk of liver cancer, which is usually fatal.

Genital Warts and Cervical Cancer

Infections with the human papillomavirus (HPV) are very common. HPV may cause genital warts, which are small, rough growths on the genitals. It may also cause cancer of the cervix in females. A simple test, called a PAP test, can detect cervical cancer. If the cancer is detected early, it usually can be cured with surgery. There is also a vaccine to prevent infection with HPV. The vaccine is recommended for females aged 11 to 26 years.

Lesson Summary

  • STIs are diseases caused by pathogens that spread through sexual contact. Abstinence from sexual activity and other risk behaviors is the only completely effective way to prevent the spread of STIs.
  • Bacterial STIs include chlamydia, gonorrhea, and syphilis. These diseases usually can be cured with antibiotics.
  • Viral STIs include genital herpes, hepatitis B, genital warts, and cervical cancer. These diseases cannot be cured, but some of them can be prevented with vaccines.

Lesson Review Questions


1. Describe how STIs spread.

2. What is the only completely effective way to prevent a sexually transmitted infection?

3. Identify three common STIs that are caused by bacteria.

4. Name and describe an STI caused by a virus.

Apply Concepts

5. Assume you are preparing a public service announcement (PSA) to explain to teens how and why to avoid STIs. List three facts you think it would be important to include for an informative and persuasive PSA.

Think Critically

6. Often, STIs do not cause symptoms. Why is it important to detect and treat STIs even when they do not cause symptoms?

7. Explain how a lack of symptoms might contribute to the spread of STIs.

8. Compare and contrast bacterial and viral STIs with regard to their treatment, cure, and prevention.

Points to Consider

From fertilization to old age, the human body is like a fantastic machine. It controls its own growth and development, protects itself from dangers in the outside world, and has amazing abilities to act, think, and feel. Like all living things, human beings are marvels of nature.

  • What have you learned about human beings and other organisms by reading this book?
  • Has studying biology given you a greater understanding and appreciation of the living world?

Opening image copyright by BioMedical, 2010. Used under license from Shutterstock.com.

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