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13.72: Sexually Transmitted Infections

Difficulty Level: At Grade Created by: CK-12
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What does "safe sex" truly mean?

"Safe Sex." The thought of a sexually transmitted infection should be enough to make you think about and believe in this saying.

Understanding Sexually Transmitted Infections

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.

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 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.


  • 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.


Use this resource to answer the questions that follow.

  1. List five facts about STDs.
  2. List five myths about STDs.


1. Describe how STIs spread.

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

3. 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.

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mucous membrane Epithelial tissue that lines inner body surfaces and cavities exposed to the external environment; produces mucous.
pubic lice Insect parasites that are transmitted sexually.
sexually transmitted disease (STD) Infection caused by a pathogen that spreads mainly through sexual contact; also known as a sexually transmitted infection (STI).
sexually transmitted infection (STI) Infection caused by a pathogen that spreads mainly through sexual contact; also known as a sexually transmitted disease (STD).
trichomoniasis Common sexually transmitted infection that is caused by protozoa.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Aug 28, 2016
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