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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.


  • chlamydia
  • genital herpes
  • genital warts
  • gonorrhea
  • hepatitis B
  • human papilloma virus (HPV)
  • sexually transmitted infection (STI)
  • syphilis
  • trichomoniasis


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


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.

Pubic Louse. Pubic 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



Preventing STIs


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.


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 the chapter The Immune System and Disease.

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.

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

Hepatitis B


Genital Warts and Cervical Cancer


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


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.


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?

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