17.3^PATHOGENS AND DISEASE^530^546^,,^28436^29089%

When pathogens enter body tissue, they can cause illness and sometimes death to the host. Worldwide, infectious diseases are responsible for more than 11 million deaths each year (Table 17.2).

Table 17.2
Top Infectious Diseases Worldwide
Disease Approximate Number of Deaths per Year
Pneumonia 3,884,000
HIV/AIDS 2,777,000
Diarrheal diseases 1,798,000
Tuberculosis 1,566,000
Malaria 1,272,000
Measles 611,000
Pertussis (whooping cough) 294,000
Tetanus 214,000
Meningitis 173,000
Syphilis 157,000
Many of the 618,000 deaths from liver cancer each year can be traced to viral hepatitis. Overall, infectious diseases kill more than 11 million people each year, representing nearly 19% of all deaths.
SOURCES:World Health Organization. 2004. The World Health Report 2004. Geneva: World Health Organization.

Pathogens include bacteria, viruses, fungi, protozoa, parasitic worms, and prions (Figure 17.5). Infections can occur almost anywhere in or on the body. Common types of infection include bronchitis, infection of the airways (bronchi); meningitis, infection of the tissue surrounding the brain and spinal cord; conjunctivitis, infection of the layer of cells surrounding the eyes; pharyngitis, or sore throat; pneumonia, infection of the lung; gastroenteritis, infection of the gastrointestinal tract; cellulitis, infection of the soft tissues; osteomyelitis, infection of the bones; and so on, for every tissue and organ.

Pathogens and associated infectious diseases.
Page 531

The most abundant living things on earth are bacteria, single-celled organisms that usually reproduce by splitting in two to create a pair of identical cells. Many species of bacteria feed on dead matter and play an important role in the recycling of nutrients for other organisms; other species feed on living things and may cause disease. Bacteria are often classified according to their shape: they may be bacilli (rod-shaped), cocci (spherical), spirochete (spiral-shaped), or vibrios (comma-shaped).

We harbor both helpful and harmful bacteria on our skin and in our gastrointestinal and reproductive tracts. The human colon contains friendly bacteria that produce certain vitamins and help digest nutrients. (A large portion of feces consists of bacteria.) Friendly bacteria also keep harmful bacteria in check by competing for food and resources and secreting substances toxic to pathogenic bacteria. For example, Lactobacillus acidophilus resides in the vagina and produces chemicals that kill yeast and bacteria that cause vaginal infections.

Not all bacteria found in the body are beneficial, however. Pathogenic bacteria in food or drink can disrupt the normal harmony in the intestines by invading cells or producing damaging toxins. Sexual activity can introduce pathogenic bacteria into the reproductive tract. Within the bloodstream, tissues, and organs, the human body is usually aseptic—devoid of bacteria. If bacteria find their way into these areas, infection may result.


Inflammation of the lungs, called pneumonia, may be caused by infection with bacteria, viruses, or fungi or by contact with chemical toxins or irritants. Pneumonia can be serious if the alveoli (air sacs) become clogged with fluid, thus preventing oxygen from reaching the bloodstream. Pneumonia often follows another illness, such as a cold or the flu, but the symptoms are typically more severe—fever, chills, shortness of breath, increased mucus production, and cough. Pneumonia ranks eighth among the leading causes of death for Americans; people most at risk for severe infection include those under age 2 and over age 75 and those with chronic health problems such as heart disease, asthma, or HIV. Bacterial pneumonia can be treated with antibiotics.

Pneumococcus bacteria are the most common cause of bacterial pneumonia; a vaccine is available and recommended for all adults age 65 and older and others at risk. Other bacteria that may cause pneumonia include Streptococcus pneumoniae, Chlamydia pneumoniae, and mycoplasmas. Outbreaks of infection with mycoplasmas are relatively common among young adults, especially in crowded settings such as dormitories.


56,000 Americans die from pneumonia each year.

—National Center for Health Statistics, 2008

Infection of the meninges, the membranes covering the brain and spinal cord, is called meningitis. Viral meningitis is usually mild and goes away on its own; bacterial meningitis, however, can be life-threatening and requires immediate treatment with antibiotics. Symptoms of meningitis include fever, a severe headache, stiff neck, sensitivity to light, and confusion. Before the 1990s, Haemophilus influenzae type b (Hib) was the leading cause of bacterial meningitis, but routine vaccination of children has reduced the occurrence of Hib meningitis. Today, Neisseria meningitidis and Streptococcus pneumoniae are the leading causes of bacterial meningitis.

Page 532

In the United States, about 2700 cases of meningitis are reported each year, although the actual number is probably higher. The disease is fatal in 10% of cases, and about 10–20% of people who recover have permanent hearing loss or other serious effects. Worldwide, meningitis kills about 170,000 people each year, particularly in the so-called meningitis belt in sub-Saharan Africa.

A vaccine is available, but it is not effective against all strains of meningitis-causing bacteria. The CDC recommends routine vaccination of children 11–18 years old, previously unvaccinated adolescents at high school entry, and first-year college students who live in dormitories.

Strep Throat and Other Streptococcal Infections

The streptococcus bacterium is spherical-shaped and often grows in chains. Streptococcal pharyngitis, or strep throat, is characterized by a red, sore throat with white patches on the tonsils, swollen lymph nodes, fever, and headache. It is typically spread through close contact with an infected person via respiratory droplets (sneezing or coughing). If left untreated, strep throat can develop into the more serious rheumatic fever (see Chapter 15). Other streptococcal infections include scarletina (scarlet fever), characterized by a sore throat, fever, bright red tongue, and a rash over the upper body; impetigo, a superficial skin infection most common among children; and erysipelas, inflammation of skin and underlying tissues.

A particularly virulent type of streptococcus can invade the bloodstream, spread to other parts of the body, and produce dangerous systemic illness. It can also cause a serious but rare infection of the deeper layers of the skin, a condition called necrotizing fascitis, or “flesh-eating strep.” This dangerous infection is characterized by tissue death and is treated with antibiotics and removal of the infected tissue or limb. Other species of streptococci are implicated in pneumonia, endocarditis (infection of the heart lining and valves), and serious infections in pregnant women and newborns.

Toxic Shock Syndrome and Other Staphylococcal Infections

The spherical-shaped staphylococcus bacterium often grows in small clusters. It is commonly found on the skin and in the nasal passages of healthy people. Occasionally, staphylococci enter the body and cause an infection, ranging from minor skin infections such as boils to very serious conditions such as blood infections and pneumonia. The strain known as methicillin-resistant Staphylococcus aureus (MRSA) has become the most common cause of skin infections treated in emergency rooms (see the box “MRSA: The Superbug?” for more information). This antibiotic-resistant strain causes painful skin lesions that resemble infected spider bites.

Staphylococcus aureus is also responsible for many cases of toxic shock syndrome (TSS). The bacteria produce a deadly toxin that causes shock (potentially life-threatening low blood pressure), high fever, a peeling skin rash, and inflammation of several organ systems. TSS was first diagnosed in women using highly absorbent tampons, which appear to allow the growth of staphylococci; however, about half of all cases occur in men and in women not using tampons. (See Chapter 6 for information on toxic shock syndrome as it relates to contraception.)


Caused by the bacterium Mycobacterium tuberculosis, tuberculosis (TB) is a chronic bacterial infection that usually affects the lungs. TB is spread via the respiratory route. Symptoms include coughing, fatigue, night sweats, weight loss, and fever.

Ten to 15 million Americans have been infected with, and therefore continue to carry, M. tuberculosis. Only about 10% of people with latent TB infections actually develop an active case of the disease; their immune system prevents the disease from becoming active. In the United States, active TB is most common among people infected with HIV, recent immigrants from countries where TB is endemic, and those who live in the inner cities. In 2007, about 13,300 cases of TB were reported in the United States—the lowest annual incidence of TB since incidence tracking began in 1953. Worldwide, about 2 billion people—one-third of the population—are infected with TB, and each year about 8.9 million develop active TB and more than 1.5 million die.

Many strains of tuberculosis respond to antibiotics, but only over a course of treatment lasting 6–12 months. Failure to complete treatment can lead to relapse and the development of strains of antibiotic-resistant bacteria. Of particular concern is the emergence of M. tuberculosis with extensive resistance to second-line drugs. These drugs are more toxic than first-line drugs—the drugs primarily used against bacterial infections. From 2000 to 2004, 20% of TB bacteria isolated in labs were multidrug resistant (MDR) and 2% were extensively drug resistant (XDR). XDR TB is geographically widespread, occurring even in the United States, and is a serious threat to public health.

Page 533
MRSA: The Superbug?

Staph infections are certainly nothing new; people have been dealing with minor staph infections for generations. In fact, the CDC estimates that as many as 30% of people carry the staphylococcus (staph) bacteria in their bodies. Most of us come into contact with the germ many times during our lives, usually with little or no consequences.

IN THE NEWS: MRSA: The Superbug
(Your score will be reported to your instructor)

In recent years, however, an anti biotic-resistant strain of staph—called methicillin-resistant Staphylococcus aureus (MRSA)—has changed the public's perception of staph as a relatively harmless germ. Medical experts have dubbed MRSA a “superbug” because it is highly resistant to several first-line medicines normally used to treat staph infections. These drugs include methicillin, penicillin, oxacillin, and amoxicillin, among others.

MRSA is not only virulent, it can be deadly. The CDC estimates that more Americans died from MRSA infections (18,650) than from AIDS (16,000) in 2005. That year, the CDC says that more than 94,000 Americans suffered from severe, invasive MRSA infections. Other estimates, however, say the number is much higher.

About 85% of MRSA infections affect patients in health care facilities or who have recently left a health care setting. Health care–associated MRSA victims typically have undergone an invasive surgical procedure or have an immune system weakened by illness or treatment for another disease. Experts say invasive MRSA is now the leading cause of surgical site infections, bloodstream infections, and pneumonia in hospitals and nursing homes.

Although less common, MRSA also affects people who have not been exposed to a health care facility; such infections are called community-associated MRSA (CA-MRSA) infections. CA-MRSA usually is not invasive, meaning it doesn't enter the soft tissues under the skin. Instead, most community-associated infections take the form of surface abscesses and pus-filled lesions. Even though their numbers are low, community-based infections are a growing concern because they may indicate that MRSA is gaining strength “in the wild,” making it more difficult to manage.

In the age of antibiotics, it's hard to imagine being infected by a germ that can't be killed. But epidemiologists say the overuse of antibiotics is one of the main reasons that bugs such as MRSA have become so strong. When people take antibiotics inappropriately or incorrectly, as Americans have been doing for decades, bacteria have an opportunity to adapt. As a result, they can become resistant to antibiotics and the drugs lose their effectiveness.

Doctors also say that MRSA is a powerful reminder of a valuable lesson: Wash your hands. Frequent hand washing may be the most effective way to avoid infections—not just from MRSA, but from a host of other germs. Other simple but important methods for preventing infections also apply to MRSA. They include the following:

  • When washing your hands, use lots of soap and scrub briskly for at least 15 seconds.

  • If soap and water aren't available, carry an alcohol-based hand sanitizer with you and use it often.

  • Keep your hands away from your face.

  • If you have an open wound, keep it clean, dry, and covered with a bandage.

  • Don't share items such as towels, razors, and tweezers; they can harbor germs and spread infection.

  • If you have a skin lesion that resem bles a spider bite, have it checked by a physician right away.

SOURCES: Centers for Disease Control and Prevention. 2007. Invasive MRSA (http://www.cdc.gov/ncidod/dhqp/ar_mrsa_Invasive_FS.html; retrieved February 16, 2009); Klevens,R.M., et al. 2007. Invasive methicillin-resistant Staphylococcus aureus infections in the United States. Journal of the American Medical Association 298(15): 1763–1771.
Tickborne Infections

Some diseases are transmitted via insect vectors. Lyme disease is one such infection, and it accounts for more than 95% of all reported vectorborne illness in the United States—more than 23,000 cases per year. It is spread by the bite of a tick of the genus Ixodes that is infected with the spiral-shaped bacterium Borrelia burgdorferi. Ticks acquire the spirochete by ingesting the blood of an infected animal; they can then transmit the microbe to their next host. The deer tick is responsible for transmitting Lyme disease bacteria to humans in the northeastern and north-central United States; on the Pacific Coast, the culprit is the western black-legged tick (Figure 17.6). Lyme disease has been reported in 48 states, but significant risk of infection is found in only about 100 counties in 10 states located in the northeastern and mid-Atlantic seaboard, the upper north-central region, and parts of northern California.

Deer tick (actual size).
Page 534

Symptoms of Lyme disease vary but typically occur in three stages. In the first stage, about 80% of victims develop a bull's-eye-shaped red rash expanding from the area of the bite, usually about 2 weeks after the bite occurs. The second stage occurs weeks to months later in 10–20% of untreated patients; symptoms may involve the nervous and cardiovascular systems and can include impaired coordination, partial facial paralysis, and heart rhythm abnormalities. These symptoms usually disappear on their own within a few weeks. The third stage, which occurs in about half of untreated people, can develop months or years after the tick bite and usually consists of chronic or recurring arthritis. Lyme disease can also cause fetal damage or death at any stage of pregnancy. Lyme disease is treatable at all stages, although arthritis symptoms may not completely resolve. Lyme disease is preventable by avoiding contact with ticks or by removing a tick before it has had the chance to transmit the infection.

Rocky Mountain spotted fever and typhus are caused by the rickettsias bacterium and are also transmitted via tick bites. Rocky Mountain spotted fever is characterized by sudden onset of fever, headache, and muscle pain, followed by development of a spotted rash. Ehrlichiosis, another tickborne disease, typically causes less severe symptoms.


About 25 million Americans suffer from ulcers, sores in the lining of the stomach or the first part of the small intestine (duodenum). Some ulcers are caused by long-term use of nonsteroidal anti-inflammatory drugs (NSAIDs). In addition, up to 90% of ulcers are caused by infection with Helicobacter pylori. Ulcer symptoms include gnawing or burning pain in the abdomen, nausea, and loss of appetite. If tests show the presence of H. pylori, antibiotics often cure the infection and the ulcers.

Other Bacterial Infections

The following are a few of the many other infections caused by bacteria:

One of the dangers of antibiotic overuse is the development of bacteria resistant to drugs. Cultures of E. coli (a bacterium normally present in the human intestine) in this laboratory dish are sensitive to four different types of antibiotics, as indicated by the wide circles where no bacteria are growing, but they are resistant to two other types, which have no effect on their growth.
Page 535

Bacteria responsible for foodborne illness are described in Chapter 12; Chapter 18 discusses sexually transmitted bacterial infections such as chlamydia.

Antibiotic Treatments

The body's immune system can fight off many, if not most, bacterial infections. However, while the body musters its defenses, some bacteria can cause a great deal of damage: Inflammation, caused by the gathering of white blood cells, may lead to scarring and permanently damaged tissues. To help the body deal with these infections, science and medicine have made a considerable contribution: antibiotics.


Antibiotics are both naturally occurring and synthetic substances that can kill bacteria. Most antibiotics work in a similar fashion: They interrupt the production of new bacteria by damaging some part of their reproductive cycle or by causing faulty parts of new bacteria to be made. Penicillins inhibit the formation of the cell wall when bacteria divide to form new cells. Other antibiotics inhibit the production of certain proteins by the bacteria, and still others interfere directly with the reading of genetic material (DNA) during the process of bacterial reproduction. Antibiotics are among the most widely prescribed and effective drugs.


When antibiotics are misused or overused, the pathogens they are designed to treat can become resistant to their effects. A bacterium can become resistant from a chance genetic mutation or through the transfer of genetic material from one bacterium to another. When exposed to antibiotics, resistant bacteria can grow and flourish, while the antibiotic-sensitive bacteria die off. Eventually, an entire colony of bacteria can become resistant to one or more antibiotics and can become very difficult to treat. Antibiotic-resistant strains of many common bacteria have developed, including strains of gonorrhea (an STD) and salmonellosis (a foodborne illness). One strain of tuberculosis is resistant to seven different antibiotics. Antibiotic resistance is a major factor contributing to the recent rise in problematic infectious diseases.

The more often bacteria encounter antibiotics, the more likely they are to develop resistance. Resistance is promoted when people fail to take the full course of an antibiotic or when they inappropriately take antibiotics for viral infections. Another possible source of resistance is the use of antibiotics in agriculture, which is estimated to account for 50–80% of the 25,000 tons of antibiotics used annually in the United States. At least four species of antibiotic-resistant bacteria are documented to have been transmitted from food animals to humans. In 2002, the FDA issued guidelines to the pharmaceutical industry regarding the use of new antibiotics in food-producing animals. Limits may be placed on the use in animals of drugs used in human medicine as last resorts for serious or life-threatening disease.

You can help prevent the development of antibiotic-resistant strains of bacteria by using antibiotics properly:

  • Don't take an antibiotic every time you get sick. They are mainly helpful for bacterial infections; they are ineffective against viruses.

  • Use antibiotics as directed, and finish the full course of medication even if you begin to feel better. This helps ensure that all targeted bacteria are killed off.

  • Never take an antibiotic without a prescription. If you take an antibiotic for a viral infection, take the wrong one, or take an insufficient dose, your illness will not improve, and you'll give bacteria the opportunity to develop resistance.


Visible only with an electron (high-magnification) microscope, viruses lack all the enzymes essential to energy production and protein synthesis in normal animal cells, and they cannot grow or reproduce by themselves. Viruses are parasites; they take what they need for growth and reproduction from the cells they invade. Once a virus is inside the host cell, it sheds its protein covering, and its genetic material takes control of the cell and manufactures more viruses like itself (Figure 17.7). In order to fight viruses, the cellular immunity system produces substances such as interferon; these same substances are also responsible for most of the symptoms of a viral illness.

Life cycle of a virus.
Page 536

Illnesses caused by viruses are the most common forms of contagious disease. Different viruses affect different kinds of cells, and the seriousness of the disease they cause depends greatly on which kind of cell is affected. The viruses that cause colds, for example, attack upper respiratory tract cells, which are constantly cast off and replaced; the disease is therefore mild. Poliovirus, in contrast, attacks nerve cells that cannot be replaced, and the consequences, such as paralysis, are severe. HIV infection, a viral illness that destroys immune system cells, can destroy the body's ability to fight infectious diseases (see Chapter 18).

The Common Cold

Although generally brief, lasting only 1–2 weeks, colds are nonetheless irritating and often interfere with one's normal activities. A cold may be caused by any of more than 200 different viruses that attack the lining of the nasal passages; rhinoviruses and coronaviruses cause a large percentage of all colds among adults.

Cold viruses are almost always transmitted by hand-to-hand contact. To lessen your risk of contracting a cold, wash your hands frequently; if you touch someone else, avoid touching your face until after you've washed your hands. Colds are not caused by exposure to cold weather or by being chilled or overheated; more colds occur in the fall and winter months, probably because of the opening of school season (children contract more colds than adults) and because people spend more time indoors in the fall and winter months, making person-to-person transmission of viruses more likely.

Page 537

If you catch a cold, over-the-counter cold remedies may help treat your symptoms but do not directly attack the viral cause (see the box “Preventing and Treating the Common Cold”). Sometimes it is difficult to determine whether your symptoms are due to a virus (as for colds, flu, and some sinus infections), a bacterium (as for other sinus infections), or an allergy, but this information is important for appropriate treatment (Table 17.3 on p. 538). For example, antibiotics will not help treat a cold but will help treat a bacterial sinus infection.

Table 17.3
What's Causing My Symptoms?
Symptoms Influenza Common Cold Allergy Sinusitis
Headache Usually Occasionally Occasionally Usually
Muscle aches Usually (severe) Usually (mild) Rarely Rarely
Fatigue, weakness Usually (severe; sudden onset; may last several weeks) Usually (mild) Rarely Rarely
Fever Usually (high, typically 102–104°F; sudden onset; lasts 3–4 days) Occasionally (mild) Never Occasionally
Cough Usually (often severe) Occasionally Occasionally Usually
Runny, stuffy nose Occasionally Usually Usually Usually (stuffy)
Nasal discharge Occasionally Usually (thick, clear to yellowish green) Usually (watery, clear) Usually (thick, yellowish green)
Sneezing Occasionally Occasionally Usually Rarely
Sore throat Occasionally Usually Occasionally Rarely
Itchy eyes, nose, throat Rarely Rarely Usually Never
SOURCE: Is it the flu? 2000. Consumer Reports, November. Copyright © 2000 by Consumers Union of U.S., Inc. Yonkers, NY 10703-1057, a nonprofit organization. Reprinted with permission from the November 2000 issue of Consumer Reports for educational purposes only. No commercial use or reproduction permitted. http://www.consumerreports.org/oh/index.htm.
Preventing and Treating the Common Cold

Colds are usually spread by hand-to-hand contact with another person or with objects such as doorknobs and telephones, which an infected person may have handled. The best way to avoid transmission is to wash your hands frequently with warm water and soap. Keeping your immune system strong is another good prevention strategy (see the guidelines provided later in the chapter).

Home Treatments
  • Get some extra rest. It isn't usually necessary to stay home in bed, but you will need to slow down a little from your usual routine to give your body a chance to fight the infection.

  • Drink plenty of liquids to prevent dehydration. Hot liquids such as herbal tea and clear chicken soup will soothe a sore throat and loosen secretions; gargling with a glass of slightly salty water may also help. Avoid alcoholic beverages when you have a cold.

  • Hot showers or the use of a humidifier can help eliminate nasal stuffiness and soothe inflamed membranes.

Over-the-Counter Treatments

Avoid multisymptom cold remedies. Because these products include drugs to treat symptoms you may not even have, you risk suffering from side effects from medications you don't need. It's better to treat each symptom separately:

  • Analgesics—aspirin, acetaminophen (Tylenol), ibuprofen (Advil or Motrin), and naproxen sodium (Aleve)—all help lower fever and relieve muscle aches. Use of aspirin is associated with an increased risk of a serious condition called Reye's syndrome in children and teenagers; for this reason, aspirin should be given only to adults.

  • Decongestants shrink nasal blood vessels, relieving swelling and congestion. However, they may dry out mucous membranes in the throat and make a sore throat worse. Nasal sprays shouldn't be used for more than 2–3 days to avoid rebound congestion.

  • Cough medicines may be helpful when your cough is non-productive (not bringing up mucus) or if it disrupts your sleep or work. Expectorants make coughs more productive by increasing the volume of mucus and decreasing its thickness, thereby helping remove irritants from the respiratory airways. Suppressants (antitussives) reduce the frequency of coughing.

  • Antihistamines decrease nasal secretions caused by the effects of histamine, so they are much more useful in treating allergies than colds. Caution: Many antihistamines can make you drowsy.

Antibiotics will not help a cold unless a bacterial infection such as strep throat is also present, and overuse of antibiotics leads to the development of drug resistance. The jury is still out on whether other remedies, including zinc gluconate lozenges, echinacea, and vitamin C, will relieve symptoms or shorten the duration of a cold. Researchers are also studying antiviral drugs that target the most common types of cold viruses.

Sometimes a cold leads to a more serious complication, such as bronchitis, pneumonia, or strep throat. If a fever of 102°F or higher persists, or if cold symptoms don't get better after 2 weeks, see your physician.


Commonly called the flu, influenza is an infection of the respiratory tract caused by the influenza virus. (Many people use the term “stomach flu” to describe gastrointestinal illnesses, but these infections are actually caused by organisms other than influenza viruses). Compared to the common cold, influenza is a more serious illness, usually including a fever and extreme fatigue. Most people who get the flu recover within 1–2 weeks, but some develop potentially life-threatening complications, such as pneumonia. The highest rates of infection occur in children. Influenza is highly contagious and is spread via respiratory droplets (see the box “The Next Influenza Pandemic—When, Not If?” on p. 539).

The most effective way of preventing the flu is through annual vaccination. The influenza vaccine consists of killed virus and provides protection against the strains of the virus currently circulating; it is updated each year in response to changes in the virus. Vaccination can be appropriate for anyone age 6 months or older who wants to reduce his or her risk of the flu. The CDC strongly recommends vaccination for all children age 6 months to 18 years, people age 50 or older, anyone age 6 months or older with long-term health conditions, anyone with a weakened immune system, women who will be pregnant during the flu season, residents of nursing homes and long-term care facilities, health care workers, and household contacts and caregivers of children up to 5 years old and persons at high risk. In addition to the injected vaccine, there is a nasal aerosol vaccine called FluMist. In 2007, the FDA approved FluMist for healthy children age 24–59 months and for nonpregnant healthy persons age 5–49 years.

Page 538

A number of medications are used to treat influenza, but in most cases they can shorten the duration of illness by little more than a day and then only if treatment begins within 1–2 days after onset of symptoms. Several medications are also effective in reducing the risk of illness from influenza; however, they are less effective than the vaccine.


200,000 Americans are hospitalized each year due to flu complications; 36,000 die from the flu.

—CDC, 2008
Measles, Mumps, and Rubella

Three childhood viral illnesses that have waned in the United States due to effective vaccines are measles, mumps, and rubella (German measles). Measles and rubella are generally characterized by rash and fever. Measles can occasionally cause more severe illness, including liver or brain infection or pneumonia; worldwide, more than 600,000 people die each year from measles. Measles is a highly contagious disease, and prior to the introduction of vaccines, more than 90% of Americans contracted measles by age 15.

Rubella, if it infects a pregnant woman, can be transmitted to a fetus, causing miscarriage, stillbirth, and severe birth defects, including deafness, eye and heart defects, and mental impairment.

Mumps generally causes swelling of the parotid (salivary) glands, located just below and in front of the ears. This virus can also cause meningitis and, in males, inflammation of the testes.

Chicken Pox, Cold Sores, and Other Herpesvirus Infections

The herpesviruses are a large group of viruses. Once infected, the host is never free of the virus. The virus lies latent within certain cells and becomes active periodically, producing symptoms. Herpesviruses are particularly dangerous for people with a depressed immune system, as in the case of HIV infection. The family of herpesviruses includes the following:

The Next Influenza Pandemic—When, Not If?
Influenza Virus Basics

There are three main types of influenza viruses, designated A, B, and C. Influenza C usually causes only mild illness and has not been associated with widespread outbreaks. Types A and B, however, are responsible for epidemics of respiratory illness that occur almost every winter. Influenza A viruses are further divided into subtypes, based on differences in two surface antigens: hemagglutinin (H) and neuraminidase (N).

IN THE NEWS: The Next Influenza Pandemic�When, Not If?
(Your score will be reported to your instructor)

Influenza A viruses also differ from types B and C in that they can infect a variety of animals in addition to humans. Aquatic birds, such as ducks, are a natural reservoir for influenza A; they carry and can spread the virus but do not themselves become ill. Avian strains can infect and cause serious illness in domestic poultry and may also infect pigs, humans, and other mammals.

Antigenic Drift and Shift

Through replication errors and gene sharing, influenza viruses undergo constant change, enabling them to evade the immune system and thereby make people susceptible to influenza throughout life. A person infected with influenza does develop antibodies, but as the H and N antigens change, the antibodies no longer recognize the virus, and rein-fection can occur.

Small changes in H and N antigens are referred to as antigenic drift; these changes are why the flu vaccine is reformulated each year. Fortunately, if the changes in H and N are small, the immune system may at least partially recognize the virus, giving many people some immune protection against the new strain.

Occasionally, an influenza A virus undergoes a sudden, dramatic change, called antigenic shift. Antigenic shift occurs when an avian influenza virus mixes and exchanges genes with a human virus; this mixing may occur when a human or an animal such as a pig is simultaneously infected by both human and avian strains of influenza A. If this occurs and the new virus spreads easily from person to person, a worldwide epidemic, called a pandemic, can occur because few people have any antibody protection against the virus.

Influenza pandemics usually occur about every 30 years or so. During the twentieth century, three major influenza pandemics occurred in humans:

  • 1918–1919 (“Spanish flu”): About 20–40% of the world's population became ill, and as many as 40 million people died, including more than 500,000 Americans.

  • 1957–1958 (“Asian flu”)

  • 1968–1969 (“Hong Kong flu”)

The Next Pandemic

Many experts believe that an influenza pandemic is overdue, inevitable, and possibly imminent. Conditions that allow the mingling of flu viruses—including wild and domestic birds, humans, and other flu carriers living in crowded conditions and close proximity—exist in many parts of the world.

Scientists have been monitoring the progress of a strain of avian influenza A(H5N1) in Asia that has caused a small but deadly number of cases in humans. The initial outbreak in 1997–1998 killed 6 people in Hong Kong. Local authorities contained the outbreak quickly by tracing its source to infected chickens, ducks, and geese and then ordering the slaughter of all domestic poultry. Many experts believe that this quick action probably averted a pandemic.

The H5N1 strain doesn't pass easily to or between humans. However, when it infects humans, it is deadly. Through mid-2006, outbreaks of H5N1 avian influenza were reported among migratory birds and poultry flocks in several countries in Asia, Africa, the Middle East, and Europe, with 63% of cases occurring in Indonesia and Vietnam. Where those outbreaks occurred, people who came into close contact with infected birds became ill or died. As of February 11, 2009, 407 people worldwide had been diagnosed with H5N1 infection and 254 of those victims died.

The more people who are infected, the greater the chance that avian and human strains will mix, producing an influenza virus that is easily transmitted between people. It is hoped that the resulting strain will be less lethal than the avian strain; human strains tend to be less deadly, and the quick death of avian hosts prevents the virus from being transmitted.

Scientists are studying the virus to determine how many mutations it would take to allow H5N1 to pass easily between humans. The avian flu virus is resistant to at least two antiviral medications—amantadine and rimantadine—which are commonly used to treat influenza. Scientists are trying to confirm whether the virus resists other currently available antiviral drugs, as well. In 2007, the Food and Drug Administration approved the first avian flu vaccine for use in humans; others are in development.

Are All Diseases Infectious?

Researchers have been identifying infectious bases for more and more diseases—even diseases that have long been thought to be caused by other factors. Some scientists feel that the role of infectious agents in the major killers of today—CVD, cancer, diabetes, and so on—has been greatly underestimated. The following are just a few examples:

  • Type 1 diabetes. A viral infection is thought to trigger the immune system to destroy insulin-producing cells in the pancreas; when this occurs, the pancreas is no longer able to produce enough insulin to metabolize glucose.

  • Multiple sclerosis (MS). Infection with any of several viruses has been proposed as the precipitating cause of MS, a condition characterized by damage to nerve fiber coverings and progressive muscle weakness.

  • Schizophrenia. There is evidence that some schizophrenic cases are associated with occurrence of prior infection. A recent study found that maternal respiratory infection increases the risk for schizophrenia in offspring from three- to sevenfold.

  • Childhood obsessive-compulsive disorder (OCD). Some cases of OCD that begin in early childhood have been found to follow infection with streptococcus bacteria. It is thought that antibodies produced to fight the infection, rather than the infection itself, may cause OCD. Some affected children improve when they are given intravenous immunoglobulin or undergo plasma exchange to remove the antibodies from their blood.

  • Heart disease. Several infectious agents have been implicated in the inflammatory processes underlying atherosclerosis, including Chlamydia pneumoniae and Cytomegalovirus. Research is under way to determine if antibiotics can lessen the risk of heart attack in infected individuals.

  • Cancer. A number of infections have been linked to specific types of cancer: human papillomavirus and cervical cancer; Epstein-Barr virus and Burkitt's lymphoma, nasopharyngeal cancer and some B-cell lymphomas; hepatitis B and C viruses and liver cancer; HTLV-1 and T-cell leukemia; Helicobacter pylori and stomach cancer; and human herpesvirus 8 and Kaposi's sarcoma.


95% of adult Americans have been infected with Epstein-Barr virus.

—CDC, 2008

Two herpesviruses that can cause severe infections in people with a suppressed immune system are cytomegalovirus (CMV), which infects the lungs, brain, colon, and eyes, and human herpesvirus 8 (HHV-8), which has been linked to Kaposi's sarcoma.

Viral Encephalitis

HSV type 1 is a possible cause of viral encephalitis, inflammation of brain tissue due to a viral infection. Other possible causes include HIV and several mosquitoborne viruses, including Japanese encephalitis virus, equine encephalomyelitis virus, and West Nile virus. Mild cases of encephalitis may cause fever, headache, nausea, and lethargy; severe cases are characterized by memory loss, delirium, diminished speech function, and seizures, and they may result in permanent brain damage or death.

Viral Hepatitis

Viral hepatitis is a term used to describe several different infections that cause inflammation of the liver. Hepatitis is usually caused by one of the three most common hepatitis viruses.

Body art procedures, such as tattooing, can lead to infection unless they are done with care.
Page 541

There are effective vaccines for hepatitis A and B, but more than 150,000 new cases of hepatitis occur in the United States each year.

Symptoms of acute hepatitis infection can include fatigue, jaundice, abdominal pain, loss of appetite, nausea, and diarrhea. Most people recover from hepatitis A within a month or so. However, 5–10% of people infected with HBV and 85–90% of people infected with HCV become chronic carriers of the virus, capable of infecting others for the rest of their lives. Some chronic carriers remain asymptomatic, while others slowly develop chronic liver disease, cirrhosis, or liver cancer. An estimated 4 million Americans and 500 million people worldwide may be chronic carriers of hepatitis. Each year in the United States, HBV and HCV are responsible for more than 15,000 deaths.

The extent of HCV infection has only recently been recognized, and most infected people are unaware of their condition. To ensure proper treatment and prevention, testing for HCV may be recommended for people at risk, including people who have ever injected drugs (even once), who received a blood transfusion or a donated organ prior to July 1992, who have engaged in high-risk sexual behavior, or who have had body piercing, tattoos, or acupuncture involving unsterile equipment (see the box “Tattoos and Body Piercing” on p. 542). Antiviral drugs are available to treat chronic hepatitis, but they are not completely effective and may have significant side effects.


An infectious viral disease that affects the nervous system, poliomyelitis (polio) can cause irreversible paralysis and death in some affected individuals. As with other vaccine-preventable diseases, the incidence of polio declined dramatically in the United States following the introduction of the vaccine, and North and South America are now considered free of the disease.


Polio remains endemic in only 4 countries: Afghanistan, India, Nigeria, and Pakistan.

—World Health Organization, 2008

Caused by a rhabdovirus, rabies is a potentially fatal infection of the central nervous system that is most often transmitted through an animal bite. U.S. rabies-related deaths among humans declined dramatically during the twentieth century due to the widespread vaccination of domestic animals and the development of a highly effective vaccine regimen that provides immunity following exposure (post-exposure prophylaxis, or PEP).

Although rabies is rare in the United States, most recent cases have been traced to bats. The CDC recommends that PEP be considered for anyone who has had direct contact with a bat, including someone who has been in the same room with a bat and who might be unaware that contact has occurred (a sleeping child, for example). PEP consists of one dose of immunoglobulin and five doses of rabies vaccine over a 28-day period.

Human Papillomavirus (HPV)

The more than 100 different types of HPV cause a variety of warts (non-cancerous skin tumors), including common warts on the hands, plantar warts on the soles of the feet, and genital warts around the genitalia. Depending on their location, warts may be removed using over-the-counter preparations or professional methods such as laser surgery or cryosurgery. Because HPV infection is chronic, warts can reappear despite treatment. As described in Chapter 16, HPV causes the majority of cases of cervical cancer. A vaccine was approved in 2006 and is recommended for girls age 11–12; it may be given to girls as young as 9 and women through age 26.

Page 542
Tattoos and Body Piercing

Because tattooing and body piercing involve the use of needles, they carry health risks. If you are considering either procedure, you can reduce the risks by carefully choosing a body artist and following aftercare directions.

Tattoos are permanent marks applied with an electrically powered instrument that injects dye into the second layer of the skin. Pain and a little bleeding are common; a tattoo typically takes a week or two to heal and should be protected from sun exposure until then.

In piercing, the artist pushes a needle through the skin; a piece of jewelry holds the piercing open. Earlobe piercing is the most common, but people also pierce the upper ear, eyebrow, tongue, lip, nose, navel, nipples, and genitals. Healing time varies depending on the site of the piercing and other factors. Some pain and swelling are common; there may be prolonged bleeding following oral piercing because the tongue contains so many blood vessels.

Potential Health Issues
  • Infection: There is a risk of transmission of bloodborne infectious agents, such as hepatitis and HIV, if instruments are not sterilized properly. No cases of HIV infection have been traced to tattooing or piercing. In 2006, the CDC reported an outbreak of methicillin-resistant Staphylococcus aureus among customers of tattoo parlors in several states. In most cases, investigators found that the tattooists had not followed proper hygiene procedures, such as changing gloves between customers. Due to the potential risks of infection, people currently cannot donate blood for 12 months following application of body art, including tattoos and some body piercings. People with heart valve problems should check with a physician prior to body piercing to determine if they should take antibiotics before the procedure.

  • Allergic reactions: Some people may be allergic to pigments used in tattooing or to metals used in body-piercing jewelry. All jewelry should be of noncorrosive materials such as stainless steel or titanium; avoid jewelry that contains nickel.

  • Nodules and scars: Some people may develop granulomas (nodules) or keloids (a type of scar) following tattooing or body piercing.

  • Problems relating to placement: Tattoos may become swollen or burned if the wearer undergoes magnetic resonance imaging (MRI), and tattoos may also interfere with the quality of MRI images. Oral ornaments may obscure dental problems in dental X rays; they may also damage teeth and fillings and interfere with speech and chewing. Navel piercings may become infected more easily because tight-fitting clothes allow moisture to collect in the area.

Tattoos are meant to be permanent and so are expensive and very difficult (or impossible) to remove completely. Tattoo removal may involve scraping or cutting off the layers of tattooed skin or using laser surgery to break up the pigment in the tattoo; some scarring can occur. Body piercings may close and heal once the jewelry is removed, but they may leave a permanent scar.

Choosing a Body Artist and Studio

A body art studio should be clean and have an autoclave for sterilizing instruments. Needles should be sterilized and disposable; piercing guns should not be used, as they cannot be adequately sterilized. The body artist should wear disposable latex gloves throughout the procedure. Leftover tattoo ink should be thrown away and not reused. Ask to see references and aftercare instructions beforehand.

Some states and local health departments regulate body art facilities. Also, ask if the studio and/or artist are members of the Alliance of Professional Tattooists (http://www.safe-tattoos.com) or the Association of Professional Piercers (http://www.safepiercing.org); these organizations have developed infection-control and other guidelines for their members to follow.

Treating Viral Illnesses

Antiviral drugs typically work by interfering with some part of the viral life cycle; for example, they may prevent a virus from entering body cells or from successfully reproducing within cells. Antivirals are currently available to fight infections caused by HIV, influenza, herpes simplex, varicella-zoster, HBV, and HVC. Most other viral diseases must simply run their course.


A fungus is an organism that absorbs food from organic matter. Fungi may be multicellular (like molds) or unicellular (like yeasts). Mushrooms and the molds that form on bread and cheese are all fungi. Only about 50 fungi out of many thousands of species cause disease in humans, and these diseases are usually restricted to the skin, mucous membranes, and lungs. Some fungal diseases are extremely difficult to treat because some fungi form spores, an especially resistant dormant stage of the organism.

Candida albicans is a common fungus found naturally in the vagina of most women. When excessive growth occurs, the result is itching and discomfort, commonly known as a yeast infection. Factors that increase the growth of C. albicans include the use of antibiotics, clothing that keeps the vaginal area excessively warm and moist, pregnancy, oral contraceptive use, and certain diseases, including diabetes and HIV infection. The most common symptom is usually a thick white or yellowish discharge. Prescription and OTC treatments are available. Women should not self-treat unless they are certain from a past medical diagnosis that they have a yeast infection. (Misdiagnosis could mean that a different and more severe infection goes untreated.) C. albicans overgrowth can occur in other areas of the body, especially in the mouth in infants (a condition known as thrush).

Page 543

Other common fungal conditions, including athlete's foot, jock itch, and ringworm, affect the skin. These three conditions are usually mild and easy to cure.

Fungi can also cause systemic diseases that are severe, life-threatening, and extremely difficult to treat. Histoplasmosis, or valley fever, causes pulmonary and sometimes systemic disease and is most common in the Mississippi and Ohio River Valleys. Coccidioidomycosis is also known as valley fever because it is most frequent in the San Joaquin Valley of California. Fungal infections can be especially deadly in people with an impaired immune system.


Another group of pathogens is single-celled organisms known as protozoa. Millions of people in developing countries suffer from protozoal infections.

Malaria, caused by a protozoan of the genus Plasmodium, is characterized by recurrent attacks of severe flu-like symptoms (chills, fever, headache, nausea, and vomiting) and may cause anemia. The protozoan is injected into the bloodstream via a mosquito bite. Although relatively rare in the United States, malaria is a major killer worldwide; each year, there are 350–500 million new cases of malaria and more than 1 million deaths, mostly among infants and children. Drugs are available to prevent and treat malaria, but in the poorest, most remote areas, conditions make it difficult to distribute drugs. Drug-resistant strains of malaria have emerged, requiring new medicines.


In developing countries, more than 10% of children's deaths are due to malaria.

—CDC, 2007

Giardiasis is caused by Giardia lamblia, a single-celled parasite that lives in the intestines of humans and animals. Giardiasis is characterized by nausea, diarrhea, bloating, and abdominal cramps, and it is among the most common waterborne diseases in the United States. People may become infected with Giardia if they consume contaminated food or water or pick up the parasite from the contaminated surface of an object such as a bathroom fixture, diaper pail, or toy. People at risk include child care workers, children who attend day care, international travelers, and hikers and campers who drink untreated water. Giardiasis is rarely serious and can be treated with prescription medications.

Other protozoal infections include the following:

Parasitic Worms

The parasitic worms are the largest organisms that can enter the body to cause infection. The tapeworm, for example, can grow to a length of many feet. Worms, including intestinal parasites such as the tapeworm and hookworm, cause a variety of relatively mild infections. Pinworm, the most common worm infection in the United States, primarily affects young children. Pinworms are white and about the size of a staple and live in the rectum of humans; they can cause itching and difficulty sleeping. Smaller worms known as flukes infect organs such as the liver and lungs and, in large numbers, can be deadly. Worm infections generally originate from contaminated food or drink and can be controlled by careful attention to hygiene.

Page 544

In recent years, several fatal degenerative disorders of the central nervous system have been linked to prions, or proteinaceous infectious particles. Unlike all other infectious agents, prions appear to lack DNA or RNA and to consist only of protein; their presence in the body does not trigger an immune response. Prions have an abnormal shape and form deposits in the brain. They may spread by triggering normal proteins to change their structure to the abnormal, damaging, form.

Prions are associated with a class of diseases known as transmissible spongiform encephalopathies (TSEs), which are characterized by spongelike holes in the brain; symptoms of TSEs include loss of coordination, weakness, dementia, and death. Known prion diseases include Creutzfeldt-Jakob disease (CJD) in humans; bovine spongiform encephalopathy (BSE), or mad cow disease, in cattle; and scrapie in sheep. Some prion diseases are inherited or the result of spontaneous genetic mutations, whereas others are the result of eating infected tissue or being exposed to prions during medical procedures such as organ transplants. A variant form of CJD referred to as vCJD occurs in humans who are infected by eating beef from cows with BSE.

As of 2006, there had been 200 cases of vCJD in the world, with 164 of those cases occurring in the United Kingdom. The first U.S. BSE case was identified in 2003. Several steps have been taken or proposed to reduce the number of BSE-infected cows and the likelihood of meat from an infected animal entering the human food supply. These include increased surveillance, limiting or banning the use of cattle products in feed for other cattle, prohibiting meat imports from countries with BSE-infected cattle, restricting certain people from giving blood, and banning the use of downer (nonambulatory disabled) cattle for food for human consumption. Prions present special challenges because they are resistant to heat, radiation, and chemicals that kill other pathogens and because the diseases they cause have a long incubation period. In fact, experts say BSE can take 50 years to incubate. Authorities are now concerned that many more people may harbor BSE than estimated, leading to an epidemic of vCJD in coming decades. Scientists are working to develop new tests that would detect low levels of dangerous prions in asymptomatic cattle.

Emerging Infectious Diseases

Emerging infectious diseases are those infections whose incidence in humans has increased or threatens to increase in the near future. They include both known diseases that have experienced a resurgence, such as tuberculosis and cholera, and diseases that were previously unknown or confined to specific areas, such as the Ebola and West Nile viruses.

Insecticides, repellants, protective clothing, and screens are helpful in reducing mosquito bites. In developing countries, mosquito nets are a first line of defense against mosquitos.
Selected Infections of Concern

Although the chances of the average American contracting an exotic infection are very low, emerging infections are a concern to public health officials and represent a challenge to all nations in the future.


A mini-outbreak of encephalitis in New York in 1999 led to identification of this virus, which had previously been restricted to Africa, the Middle East, and parts of Europe. Between 1999 and 2007, the virus spread across the United States and caused more than 27,605 illnesses and 1086 deaths. West Nile virus is carried by birds and then passed to humans when mosquitoes bite first an infected bird and then a person. Most people who are bitten have few or no symptoms, but the virus can cause permanent brain damage or death in some. Vaccines are being developed for West Nile virus, but it is important to protect yourself from mosquito bites.


In February 2003, SARS appeared in southern China and quickly spread to more than 15 countries; it is a form of pneumonia that is fatal in about 5–15% of cases. SARS is caused by a new type of coronavirus found in wildlife that may have crossed the species barrier when certain wildlife species were consumed as delicacies. It has reemerged several times since 2003, and by 2004 had been responsible for more than 8000 illnesses and 800 deaths. There have been no new cases reported since 2004.

Page 545

The leading viral cause of gastroenteritis, an intestinal inflammation that results in vomiting and diarrhea, rotavirus infects almost every child at one time or another. Worldwide, the virus kills about 600,000 children each year, mostly in developing countries. Left untreated, rotavirus-induced diarrhea can become severe and lead to dehydration, which can be fatal. Rotavirus spreads through poor hygiene and sanitation practices.


This potentially deadly strain of E. coli, transmitted in contaminated food, can cause bloody diarrhea and kidney damage. In 2006, more than 200 people became ill and 3 died from eating spinach from a field in California contaminated by cattle and wild-life. Other outbreaks have been linked to lettuce, alfalfa sprouts, unpasteurized juice, petting zoos, and contaminated public swimming pools. An estimated 70,000 cases and 61 deaths occur in the United States each year.


Since first being recognized in 1993, over 300 cases of hantavirus pulmonary syndrome (HPS) have been reported in the United States. HPS is caused by the rodentborne Sin Nombre virus (SNV) and is spread primarily through airborne viral particles from rodent urine, droppings, or saliva. It is characterized by a dangerous fluid buildup in the lungs and is fatal in about 45% of cases.


Many environmental factors contribute to the spread of infectious diseases. Here are a few examples:

  • In poverty-stricken regions, many people become ill as a result of unsanitary conditions and a lack of clean drinking water.

  • Unsustainable development practices, such as clearing forests and draining wetlands, disturb the ecosystem and force many disease-carrying vectors (such as vermin and insects) out of their natural habitats and into contact with people.

  • A shift in rainfall patterns (perhaps caused by global warming) may allow mosquito-borne diseases such as malaria to spread from the tropics into the temperate zones. Since many species of mosquito in North America can carry malaria, such changes could someday make malaria a widespread health threat to the United States.

For more information on the environment and environmental health, see Chapter 19.


Human outbreaks of the often fatal Ebola hemorrhagic fever (EHF) have occurred only in Africa. The Ebola virus is transmitted by contact with infected blood or other body secretions, and many cases of EHF have been linked to unsanitary conditions in medical facilities. Because symptoms appear quickly and 70% of victims die, usually within a few days, the virus tends not to spread widely.

Factors Contributing to Emerging Infections

What's behind this rising tide of infectious diseases? Contributing factors are complex and interrelated.


New or increasing drug resistance has been found in organisms that cause malaria, tuberculosis, gonorrhea, influenza, AIDS, and pneumococcal and staphylococcal infections. Infections caused by drug-resistant organisms prolong illness, and—if not treated in time with more effective, expensive drugs—they can cause death. Some bacterial strains now appear to be resistant to all available antibiotics.


Nearly all major bacterial infections are becoming resistant to common antibiotics.

—CDC, 2007

More than 1 billion people live in extreme poverty, and half the world's population have no regular access to essential drugs. Population growth, urbanization, overcrowding, and migration (including the movement of refugees) also spread infectious diseases.


A poor public health infrastructure is often associated with poverty and social upheaval, but problems such as contaminated water supplies can occur even in industrial countries. Inadequate vaccination has led to the reemergence of diseases such as diphtheria and pertussis. Natural disasters such as hurricanes also disrupt the public health infrastructure, leaving survivors with contaminated water and food supplies and no shelter from disease-carrying insects.


More than 500 million travelers cross national borders each year, and international tourism and trade open the world to infectious agents. SARS was quickly spread throughout the world by infected air travelers. The reintroduction of cholera into the Western Hemisphere is thought to have occurred through the discharge of bilge water from a Chinese freighter into the waters off Peru.


Food now travels long distances to our table, and microbes are transmitted along with it. Mass production of food increases the likelihood that a chance contamination can lead to mass illness.


Changes in patterns of human behavior also influence the spread of infectious diseases. The widespread use of injectable drugs rapidly transmits HIV infection and hepatitis. Changes in sexual behavior over the past 30 years have led to a proliferation of old and new STDs. The use of day-care facilities for children has led to increases in the incidence of several infections that cause diarrhea.

Page 546

The deliberate release of deadly infectious agents is an ongoing concern. In 2001, infectious anthrax spores sent through the mail sickened 11 and killed 5 people in the United States. Potential bioterrorism agents that the CDC categorizes as a highest concern are those that can be easily disseminated or transmitted from person to person and that have a high mortality rate and the potential for a major public health impact; these include anthrax, smallpox, plague, botulism, and viral hemorrhagic fevers such as Ebola.

Other Immune Disorders: Cancer and Autoimmune Diseases

The immune system has evolved to protect the body from invasion by foreign microorganisms. Sometimes, as in the case of cancer, the body comes under attack by its own cells. As explained in Chapter 16, cancer cells cease to cooperate normally with the rest of the body and multiply uncontrollably. The immune system can often detect cells that have recently become cancerous and then destroy them just as it would a foreign microorganism. But if the immune system breaks down, as it may when people get older, when they have certain immune disorders (including HIV infection), or when they are receiving chemotherapy for other diseases, the cancer cells may multiply out of control before the immune system recognizes the danger. By the time the immune system gears up to destroy the cancerous cells, it may be too late.

Another immune disorder occurs when the body confuses its own cells with foreign organisms. As described earlier, the immune system must recognize many thousands of antigens as foreign and then be able to recognize the same antigens again and again. Our own tissue cells also are antigenic; that is, they would be recognized by another person's immune system as foreign. A delicate balance must be maintained to ensure that one's immune system recognizes only truly foreign antigens as enemies; erroneous recognition of one's own cells as foreign produces havoc.

This is what happens in autoimmune diseases such as rheumatoid arthritis and systemic lupus erythematosus. In this type of malady, the immune system seems to be a bit too sensitive and begins to misapprehend itself as non-self. For reasons not well understood, these conditions are much more common in women than in men (see the box “Women and Autoimmune Diseases”).


Have you ever had any of the illnesses described in the preceding section? How were you exposed to the disease? Could you have taken any precautions to avoid it?