fetus
The developing individual from week 8 until birth.
gastrulation
The development of different layers of cells in the embryo; generally occurs during the second week after fertilization.
implantation
The embedding of the blastocyst in the lining of the uterus; occurs about a week after fertilization. Once implantation occurs, the blastocyst is called an embryo.
infancy
The first year of life after birth.
mesoderm
Cell layer of the embryo that forms tissues that provide movement and support; develops into cells such as muscles, bones, teeth, and blood.
morula
Initial ball of cells formed the first few days after fertilization; formed within a fallopian tube.
organogenesis
The development of specific organs within the three cell layers.
placenta
A temporary organ in which nutrients and wastes are exchanged between the mother and the embryo or fetus.
pregnancy
The carrying of one or more offspring from fertilization until birth.
trophoblast
Outer layer of cells within the blastocyst; will develop into structures which includes the placenta.
Points to Consider
Many diseases become more common as people age, but some diseases are more common in adolescents and young adults, including sexually transmitted diseases (STDs). What are examples of STDs?
How common are STDs in teens and young adults?
Why are STDs more common during these two stages of life?
Lesson 25.4: Sexually Transmitted Diseases
Lesson Objectives
Explain how STDs are transmitted and how they can be prevented.
Identify and describe three common bacterial STDs.
Identify and describe three common viral STDs.
Introduction
A sexually transmitted disease (STD) is an illness caused by a pathogen that is transmitted from one person to another mainly through sexual contact. Worldwide, as many as one million people a day become infected with STDs. The majority of these infections occur in people under the age of 25.
Sexually Transmitted Diseases
Common STDs include chlamydia, gonorrhea, syphilis, human immunodeficiency virus (HIV) infection, genital herpes, hepatitis B, and genital warts. To be considered an STD, a disease must have only a small chance of spreading naturally in ways other than sexual contact. Many diseases that can spread through sexual contact spread more commonly by other means. These diseases are not considered STDs.
Pathogens that Cause STDs
STDs may be caused by several different types of pathogens, including protozoa, insects, bacteria, and viruses.
The protozoa Trichomonas vaginalis causes an STD called trichomoniasis. This is an infection of the vagina in females and the urethra in males.
Pubic lice, like the one in Figure below, are insect parasites that can be transmitted sexually. They suck the blood of their host and irritate the skin in the pubic area.
Figure 25.18
A magnified pubic louse ().
Although these STDs are common, the majority of STDs are caused by bacteria or viruses. Several bacterial and viral STDs are described below. It is important to note that most bacterial STDs can be cured with antibiotics, whereas viral STDs do not have cures, although some can be prevented with vaccines.
How STDs Spread
Most of the pathogens that cause STDs 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 STDs can also be transmitted through body fluids such as blood, semen, and breast milk. For example, in the past, HIV and hepatitis B were transmitted through blood transfusions. This no longer occurs because donated blood is now screened for the pathogens. Use of shared injection or tattoo needles is another way in which blood and pathogens can be transferred from one person to another. A number of STDs can also be transmitted from a mother to her baby through her blood during childbirth or through her breast milk after birth.
STDs are much more common in young adults and teens than in older people. One reason is that young people are more likely to take risks and to think “It can’t happen to me.” They also may not know how STDs are spread. In addition, younger people may be more sexually active than older people.
Preventing STDs
The only completely effective way to prevent infection with STDs is to avoid sexual activity and other known risk behaviors, such as using contaminated needles. Using condoms can decrease the risk of contracting STDs during some types of sexual activity. However, using condoms is not a foolproof method. Pathogens may be present on areas of the body not covered by condoms. Condoms can also break or be used incorrectly.
Bacterial STDs
Many STDs are caused by bacteria. Some of the most common bacterial STDs are chlamydia, gonorrhea, and syphilis.
Chlamydia
Chlamydia is an STD caused by the bacterium Chlamydia trachomatis. It is the most common STD in the U.S. Each year, about four million new chlamydia infections occur in Americans. As shown in Figure below, females are much more likely to develop chlamydia than males. This figure also shows how common this STD is in teens and young adults compared with older people. This is typical of most STDs.
Figure 25.19
This bar graph shows the number of cases of chlamydia per 100,000 people in the United States in 2004, by age and sex. Chlamydia rates were greatest from both sexes between the ages of 15 and 34 years. The rates in females at all ages were much greater than the rates in same-aged males.
Symptoms of chlamydia may include a burning sensation during urination and a discharge from the vagina or penis. Chlamydia can be cured with antibiotics. However, in the majority of cases, there are no symptoms. As a result, many people are not aware they are infected and do not seek treatment.
It is important to detect and treat chlamydia infections even when they do not cause symptoms. Untreated chlamydia can lead to more serious problems, especially in females. Almost half of all women with untreated chlamydia develop pelvic inflammatory disease (PID), which is an infection of the uterus, Fallopian tubes, and/or ovaries. PID can lead to scarring of the reproductive organs, which may cause pain and difficulty becoming pregnant. Chlamydia causes an estimated half million cases of PID in the U.S. each year.
In addition to sexual transmission, chlamydia can be passed from a woman to her baby before or during birth. Before birth, chlamydia infection of the fetus may cause the fetus to be born too soon. During birth, a baby’s eyes can become infected with the bacteria. If the eye infection is not treated, it can lead to blindness. Because chlamydia is common and often symptomless, newborns are treated routinely with eye drops to prevent chlamydia eye infections from developing.
Gonorrhea
Gonorrhea is an STD caused by the bacterium Neisseria gonorrheae. Gonorrhea is also a common STD. In the U.S., an estimated 700,000 people are infected with gonorrhea each year.
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 the most commonly used antibiotics. Gonorrhea infections may not cause symptoms, especially in females, so they often go untreated. Untreated gonorrhea can lead to PID in females. In males, it can lead to inflammation of the epididymis, prostate, and urethra.
Gonorrhea can be passed from an infected woman to her baby during childbirth. This may cause an eye infection. The infection must be treated promptly to prevent blindness.
Syphilis
Syphilis is an STD caused by the bacterium Treponema pallidum. In the U.S., about 70,000 new cases of syphilis occur each year. Syphilis is less common than either chlamydia or gonorrhea, but it
is more serious if it is not treated. Untreated syphilis can even cause death.
Early symptoms of infection with syphilis include the development of a small sore on or near the genitals. The sore is painless and heals on its own, so it may go unnoticed. Many people do not realize they have become infected until much later, so they do not seek treatment. If diagnosed and treated early, most cases of syphilis can be cured with antibiotics. However, if syphilis goes untreated, the disease may progress through the stages shown in Table below. Untreated syphilis can eventually cause serious damage to the heart, brain, and other organs.
Stages of Syphilis Infection Stage Time After Initial Infection Signs and Symptoms
Primary 2 days Small sore on genitals
Secondary 1–6 months Rash, fever, sore throat, headache
Latent 6–12 months None
Tertiary 1–10 years Chronic inflammation, damage to aorta and heart, narrowing of arteries, stroke, meningitis, muscle weakness
Viral STDs
STDs caused by viruses include genital herpes, hepatitis B, genital warts, and cancer of the cervix. Another common viral STD is HIV infection, which causes acquired immune deficiency syndrome, or AIDS. HIV and AIDS are described in the chapter titled Immune System and Disease.
Genital Herpes
Genital herpes is an STD caused by herpes simplex virus type 2 (HSV-2). In the U.S., as many as 20% of males and 25% of females may be infected with HSV-2. The virus is closely related to herpes simplex virus type 1 (HSV-1), which causes cold sores on the lips. Both viruses are transmitted by direct contact. Both also cause similar symptoms, except HSV-2 infects the genitals instead of the mouth.
Symptoms of genital herpes include painful, fluid-filled blisters on the penis, vulva, or nearby membranes (Figure below). The initial infection soon clears up on its own. However, herpes virus particles travel to local nerves, where they evade the immune system and remain for the life of the infected person. Periodically, some of the virus particles travel back to the skin and cause new outbreaks of blisters. Outbreaks may be triggered by stress or other factors. A person with genital herpes is most likely to transmit the virus during an outbreak.
Figure 25.20
Genital herpes causes outbreaks of fluid-filled blisters, like ones shown here, on the membranes of reproductive organs.
There is no known cure for genital herpes. Once a person becomes infected, there is no way to eradicate the virus from the body. However, antiviral drugs can prevent outbreaks or reduce their length and severity. The drugs also reduce the risk of transmitting the virus. A vaccine to prevent infections with HSV-2 may soon be available.
Genital herpes may cause emotional problems because it affects intimate relationships throughout a person’s life. However, it is not considered to be a serious disease from the standpoint of physical health. On the other hand, herpes is very serious for newborns if they are infected with the virus during childbirth. It can lead to blindness, mental retardation, and even death.
Hepatitis B
Hepatitis B is inflammation of the liver caused by infection with the hepatitis B virus. In the U.S., there are about 200,000 new cases of hepatitis B diagnosed each year. In addition, as many as 5,000 Americans die each year from hepatitis B infections.
Early symptoms of hepatitis B include vomiting and jaundice, which is yellowing of the skin and eyes. Hepatitis B often gets better on its own after a few weeks or months and causes no long-lasting effects. However, in a small percentage of people it develops into a chronic, or long-term, disease. In some people, chronic hepatitis B causes few if any symptoms, although people infected with the virus can still spread it to others. In other people, chronic hepatitis B causes continuous inflammation of the liver. This eventually damages the liver. It also increases the risk of liver cancer, which is usually fatal.
Hepatitis B cannot be cured. Antiviral drugs can help prevent liver damage in people with chronic hepatitis B, but they cannot eradicate the virus from the body. However, vaccines have been developed to prevent hepatitis B infection.
In addition to sexual transmission, hepatitis B is commonly transmitted through contaminated needles and from mother to child during childbirth. Newborns are much more likely than older people to develop chronic hepatitis B. This is because their immune system is immature and unable to fight off the virus.
Genital Warts and Cervical Cancer
Both genital warts and cancer of the cervix are caused by the human papillomavirus (HPV). There are more than 100 types of HPV. Some types of HPV cause common warts, which are small, rough growths on the hands, knees, or feet. These HBV viruses are transmitted by casual skin-to-skin contact. Other types of HPV cause genital warts or cervical cancer. These HPV viruses are transmitted through sexual contact. Genital HPV infections are very common. In the U.S., more than six million people become infected each year.
Many types of HPV that are transmitted sexually do not cause any noticeable symptoms. However, several types cause genital warts or cervical cancer. Cervical cancer is easily detected with a Pap test, which involves examining a sample of cervical cells for cancerous changes. If detected early, cervical cancer can be cured with surgery. Since 2006, a vaccine has been available to prevent transmission of the most common types of HPV that cause genital warts and cervical cancer. The vaccine is recommended for females from aged 11 to 26 years.
Lesson Summary
STDs are diseases caused by pathogens that spread through sexual contact. Abstinence from sexual activity is the only completely effective way to prevent the spread of STDs.
Bacterial STDs include chlamydia, gonorrhea, and syphilis. These diseases can be cured with antibiotics.
Viral STDs include genital herpes, hepatitis B, genital warts, and cervical cancer. These diseases cannot be cured, but some of them can be prevented with vaccines.
Review Questions
Describe how STDs spread.
What is the only completely effective way to prevent infection from STDs?
Identify three common STDs that are caused by bacteria.
Name and describe an STD caused by a virus.
Why is it important to treat STDs even when they do not cause symptoms?
How does lack of symptoms contribute to the spread of STDs?
Further Reading / Supplemental Links
Jeyendran, Rajasingam S., Sex, Sperm, & STDs: What Every Teenage Boy Needs to Know. iUniverse, Inc., 2006.
Stanley, Deborah, Sexual Health Information for Teens. Omnigraphics, 2003.
http://www.cdc.gov/STD/stats04/trends2004.htm
http://www.wrongdiagnosis.com/c/chlamydia/prevalence.htm
http://www.wrongdiagnosis.com/h/hepatitis_b/prevalence.htm
http://www.wrongdiagnosis.com/s/syphilis/prevalence.htm
http://www.avert.org/std.htm
http://www.cdc.gov/nchstp/dstd/disease_info.htm
http://www.kidshealth.org/teen/sexual_health/stds/std.html
http://www.4woman.org/faq/stdsgen.htm
http://en.wikipedia.org
Vocabulary
chlamydia
A STD caused by the bacterium Chlamydia trachomatis; the most common STD in the U.S. Each year, about four million new chlamydia infections occur in Americans.
genital herpes
An STD caused by herpes simplex virus type 2 (HSV-2). In the U.S., as many as 20% of males and 25% of females may be infected with HSV-2.
gonorrhea
An STD caused by the bacterium Neisseria gonorrheae. In the U.S., an estimated 700,000 people are infected with gonorrhea each year.
hepatitis B
An inflammation of the liver caused by infection with the hepatitis B virus.
pelvic inflammatory disease (PID)
An infection of the uterus, Fallopian tubes, and/or ovaries.
sexually transmitted disease (STD)
A illness caused by a pathogen that is transmitted from one person to another mainly through sexual contact.
syphilis
An STD caused by the bacterium Treponema pallidum. In the U.S., about 70,000 new cases of syphilis occur each year.
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 other living things, human beings are marvels of nature.
What have you learned about human beings and other organisms by reading this book?
Chapter 26: Appendix: Biology I
Investigation and Experimentation Activities
The following activities are based on information provided within this FlexBook or taken directly from the Teacher Edition.
Collecting and Analyzing Data
In this activity students will select and use appropriate tools and technology to perform tests, collect data, analyze relationships, and display data.
Students build skills in interpreting data and measuring through a simple activity. Have students measure a certain volume of water into a number of cups, such as 4 - 10 cups. Then have them first look online for what the threshold amount is for detecting sugar in water (see: http://www.skidmore.edu/~hfoley/Perc2.htm#ch2demo1).
CK-12 Biology I - Honors Page 126
