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Sexually Transmitted Infections

Defines and describes common bacterial and viral sexually transmitted infections.

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What is safe sex?

Safe sex is sexual activity engaged in by people who have taken precautions to protect themselves against sexually transmitted infections. Abstaining from sexual activity, however, is the only way to be absolutely sure that you won't get a sexually transmitted infection.

Sexually Transmitted Infections

A sexually transmitted infection (STI) is an infection that spreads through sexual contact. STIs are caused by pathogens, a living thing or virus that causes infection. The pathogens enter the body through the reproductive organs. Many STIs also spread through body fluids, such as blood. For example, a shared tattoo needle is one way an STI could spread. Some STIs can also spread from a mother to her baby during childbirth.

STIs are more common in teens and young adults than in older people. One reason is that young people are more likely to take risks. They also may not know how STIs spread. They are likely to believe myths about STIs (Table below).

Myth Fact
If you are sexually active with just one person, you can’t get STIs. The only way to avoid the risk of STIs is to practice abstinence from sexual activity.
If you don’t have any symptoms, then you don’t have an STI. Many STIs do not cause symptoms, especially in females.
Getting STIs is no big deal, because STIs can be cured with medicine. Only some STIs can be cured with medicine; other STIs cannot be cured.

Most STIs are caused by bacteria or viruses. STIs caused by bacteria usually can be cured with drugs called antibiotics. But antibiotics are not effective against viruses. Therefore, STIs caused by viruses are not treated with antibiotics. Other drugs may be used to help control the symptoms of viral STIs, but they cannot be cured. Once you have a viral STI, you are usually infected for life.

Bacterial STIs

In the U.S., chlamydia is the most common STI caused by bacteria. Females are more likely than males to develop the infection. Rates of chlamydia among U.S. females in 2006 are shown below (Figure below). Rates were much higher in teens and young women than in other age groups.

Chlamydia may cause a burning feeling during urination. It may also cause a discharge (leaking of fluids) from the vagina or penis. But in many cases it causes no symptoms. As a result, people do not know they are infected, so they don’t go to the doctor for help. If chlamydia goes untreated, it may cause more serious problems in females. It may cause infections of the uterus, fallopian tubes, or ovaries. These infections may leave a woman unable to have children.

This graph shows data on the number of cases of chlamydia in U.S. males and females in 2009. Which two age groups had the highest rates of chlamydia? Why do you think rates were highest in these age groups?

Gonorrhea is another common STI. Gonorrhea may cause pain during urination. It may also cause a discharge from the vagina or penis. On the other hand, some people with gonorrhea have no symptoms. As a result, they don’t seek treatment. Without treatment, gonorrhea may lead to infection of other reproductive organs. This can happen in males as well as females.

Syphilis is a very serious STI. Luckily, it is less common than chlamydia or gonorrhea. Syphilis usually begins with a small sore on the genitals. This is followed a few months later by a rash and flu-like symptoms. If syphilis is not treated, it may damage the heart, brain, and other organs. It can even cause death.

Viral STIs

Genital warts are an STI caused by human papilloma virus, or HPV. They are one of the most common STIs in teens. HPV infections cannot be cured. But a new vaccine called Gardasil® can prevent most HPV infections in females. Many doctors recommend that females between the ages of 9 and 26 years receive the vaccine. Preventing HPV infections in females is important because HPV can also cause cancer of the cervix.

A related herpes virus causes cold sores on the lips (Figure below). Both viruses cause painful blisters. In the case of genital herpes, the blisters are on the penis or around the vaginal opening. The blisters go away on their own, but the virus remains in the body. The blisters may come back repeatedly, especially when a person is under stress. There is no cure for genital herpes. But drugs can help prevent or shorten outbreaks. Researchers are trying to find a vaccine to prevent genital herpes.

This lip blister, or cold sore, is caused by a herpes virus. The virus is closely related to the virus that causes genital herpes. The genital herpes virus causes similar blisters on the genitals. If you’ve ever had a cold sore, you know how painful they can be. Genital herpes blisters are also painful.

Hepatitis B is a disease of the liver. It is caused by a virus called hepatitis B, which can be passed through sexual activity. Hepatitis B causes vomiting. It also causes yellowing of the skin and eyes. The disease goes away on its own in some people. Other people are sick for the rest of their lives. In these people, the virus usually damages the liver. It may also lead to liver cancer. Medicines can help prevent liver damage in these people. There is also a vaccine to protect against hepatitis B.

HIV stands for "human immunodeficiency virus." It is the virus that causes AIDS. HIV and AIDS are described in a previous lesson. HIV can spread through sexual contact. It can also spread through body fluids such as blood. There is no cure for HIV infection, and AIDS can cause death, although AIDS can be delayed for several years with medication. Researchers are trying to find a vaccine to prevent HIV infection.


  • chlamydia: Sexually transmitted bacterial infection; the most common STI caused by a bacteria.
  • genital herpes: Sexually transmitted infection caused by a herpes virus; characterized by periodic outbreaks of blisters on the genitals.
  • gonorrhea: Common sexually transmitted infection caused by bacteria.
  • hepatitis B: Inflammation of the liver caused by infection with hepatitis B virus; often transmitted through sexual contact.
  • HIV: Human immunodeficiency virus; virus that causes AIDS.
  • HPV: Sexually transmitted virus that causes genital warts and cervical cancer.
  • pathogen: Living thing or virus that causes disease.
  • sexually transmitted infection (STI): Infection caused by a pathogen that spreads mainly through sexual contact.
  • syphilis: Sexually transmitted infection caused by bacteria that may eventually be fatal if untreated.


  • Sexually transmitted infections (STIs) are caused by pathogens. They spread through sexual contact or other exchanges of body fluids.
  • Examples of STIs caused by bacteria include chlamydia, gonorrhea, and syphilis.
  • Examples of STIs caused by viruses include HPV, genital herpes, hepatitis B, and AIDS.


Use the resource below to answer the questions that follow.

  1. How many cases of sexually transmitted infections occur in the United States every year?
  2. What can untreated chlamydial and gonococcal infection lead to? How can this have long term effects on people?
  3. What does HPV cause?
  4. Why might living in an urban area increase a person's chance of contracting a sexually transmitted infection? Do you think this situation is different for non-sexually transmitted infections? Explain your reasoning fully.
  5. What are some ways to minimize the risk of contracting a sexually transmitted infection?


  1. It is especially important for females to be protected from HPV infections. Why is this the case?
  2. Explain why bacterial STIs are treated differently than viral STIs.

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