Biology of Humans

13a. Infectious Disease

 

In the previous chapter, we learned about the ways that our bodies protect us against pathogens. In this chapter, we discuss the most important categories of pathogens, the disease-causing organisms introduced in the previous chapter. We explore how they cause harm, how they are transmitted from person to person, and how they are studied so that steps may be taken to hold them in check.

 

Pathogens

 

As indicated in the previous chapter, pathogens are disease-causing organisms. There are different types of pathogens and a wide range of differences within each type. As a result, each pathogen has specific effects on the body, and some pathogens are a greater menace than others. In this chapter, we look at bacteria, viruses, protozoans, fungi, parasitic worms, and prions. We consider the general means these different types of pathogens use to attack the body and cause symptoms. Keep in mind, however, as we do so, that some of the symptoms are caused not by the pathogen itself but by the immune responses our body uses to protect us (Chapter 13).

Virulence is the relative ability of a pathogen to cause disease. Some factors contributing to this ability are how easily the pathogen invades tissues and the degree and type of damage it does to body cells. An organism that always causes disease—the typhoid bacterium, for instance—is highly virulent. On the other hand, the yeast Candida albicans, which sometimes causes disease, is moderately virulent.

 

Bacteria

Bacterial cells differ greatly from the cells that make up our bodies. Recall from Chapter 3 that our bodies are made up of eukaryotic cells that contain a nucleus and membrane-bound organelles. Bacteria, in contrast, are prokaryotes, which means they lack a nucleus and other membrane-bound organelles. Nearly all bacteria have a semirigid cell wall composed of a strong mesh of peptidoglycan, a type of polymer consisting of sugars and amino acids. The cell wall endows most types of bacteria with one of three common shapes: a sphere (a spherical bacterium is called a coccus) that can occur singly, in pairs, or in chains; a rod (bacillus) that usually occurs singly; or a spiral or corkscrew shape (spirilla) (Figure 13a.1).

 

 

FIGURE 13a.1. Bacteria have three basic shapes: (a) spherical (coccus), (b) rod-shaped (bacillus), and (c) corkscrew-shaped (spirilla). All bacteria are prokaryotic cells, meaning they lack a nucleus and membrane-bound organelles.

 

·       Vaccinations have helped eliminate certain infectious diseases.

Bacteria can reproduce rapidly. This rapid growth rate is a matter of concern because the greater the number of bacteria, the greater harm they can potentially do. Rapid reproduction is possible because bacteria reproduce asexually in a type of cell division called binary fission, in which the bacterial genetic material (DNA) is copied, the cell is pinched in half, and each new cell contains a complete copy of the original genetic material. Under ideal conditions, certain bacteria can divide every 20 minutes. Thus, if every descendant lived, a single bacterium could result in a massive infection of trillions of bacteria within 24 hours. If a percentage of the descendant bacteria die before dividing, the population of bacteria will begin to grow more slowly than does a population in which all descendants survive, but the populations will eventually have the same growth rate.

Bacteria have defenses or other adaptive mechanisms that affect their virulence. Some bacteria have long, whiplike structures called flagella that allow them to move and spread through tissues to new areas where they can cause infection. Bacteria may also have filaments, called pili, that help them attach to the cells they are attacking. Outside the bacterial cell, there is often a capsule that provides a means of adhering to a surface and prevents scavenger cells of the immune system (phagocytes; see Chapter 13) from engulfing them.

Bacterial enzymes and toxins. Destructive enzymes and toxins (poisons) are among the offensive mechanisms that certain bacteria use to spread and to attack. Some of these bacteria secrete enzymes that directly damage tissue and cause lesions, allowing the bacteria to push through tissues like a bulldozer. An example is Clostridium, the bacterium that causes gas gangrene, a condition in which tissue dies because its blood supply is shut off. The bacterium secretes an enzyme that dissolves the material holding muscle cells together, permitting the bacteria to spread with ease. When this bacterium digests muscle cells for energy, a gas is produced that presses against blood vessels and shuts off the blood supply. In addition, Clostridium causes anemia by secreting an enzyme that bursts red blood cells.

Most bacteria, however, do their damage by releasing toxins (poisons) into the bloodstream or the surrounding tissues. If the toxins enter the bloodstream, they can be carried throughout the body and disturb body functions.

The disease symptoms depend on which body tissues are affected by the toxin. Thus, the bacteria that cause various types of food poisoning have different effects. Staphylococcus is often found contaminating poultry, meat and meat products, and creamy foods such as pudding or salad dressing. These bacteria multiply when food is undercooked or unrefrigerated. The toxins they produce stimulate cells in the immune system to release chemicals that result in inflammation, vomiting, and diarrhea. Another type of food poisoning is caused by Salmonella, often encountered in undercooked contaminated chicken or eggs. In this case, the toxin causes changes in the permeability of intestinal cells, leading to diarrhea and vomiting. One type of Escherichia coli (E. coli) food poisoning is often caused by contaminated meat, particularly ground meat. Besides vomiting and diarrhea, E. coli toxin can cause kidney failure in children and the elderly. The toxin that causes botulism, a type of food poisoning often brought on by eating improperly canned food, is one of the most toxic substances known. Produced by the bacterium Clostridium botulinum, it interferes with nerve functioning, especially motor nerves that cause muscle contraction. Death occurs because muscle paralysis prevents breathing. If enough of it is consumed, this toxin is almost always fatal.

Beneficial bacteria. Many bacteria are beneficial. For instance, certain bacteria are important in food production, especially of dairy products such as cheese and yogurt. Other bacteria are important in the environment, serving as decomposers or driving the cycling of nitrogen, carbon, and phosphorus between organisms and the environment (see Chapter 23). Yet other bacteria are important in genetic engineering (see Chapter 21). Some bacteria are normal residents in the body that keep potentially harmful microorganisms in check.

Antibiotics. Fortunately, bacteria can be killed. As we learned in Chapter 13, the human body has its own array of defenses against foreign invaders. But when the body needs outside help, we can call on antibiotics, chemicals that inhibit the growth of microorganisms. Antibiotics work to reduce the number of bacteria or slow the growth rate of the population, allowing time for body defenses to conquer the bacteria. Some antibiotics kill bacteria directly by preventing the synthesis of bacterial cell walls, causing them to burst. Recall that our body cells lack cell walls (see Chapter 3). Thus, our cells are unaffected by antibiotics that target cell walls. Some antibiotics block protein synthesis by bacteria but do so without interfering with cell protein synthesis in human body cells. This selective action is possible because the structure of ribosomes, the organelles on which proteins are synthesized, is slightly different in bacteria and humans.

When antibiotics were introduced during the 1940s, they were considered to be miracle drugs. For the first time, there was a cure for devastating bacterial diseases such as pneumonia, bacterial meningitis, tuberculosis, and cholera. Today, there are more than 160 antibiotics. These lifesaving drugs have become so commonplace that we take them for granted.

Unfortunately, antibiotics are losing their power. Infections that were once easy to cure with antibiotics can now turn deadly as bacteria gain resistance to the drugs. Several bacterial species capable of causing life-threatening illnesses have produced strains that are resistant to every antibiotic available today.1

Contradictory as it may seem, the use of antibiotics can actually increase antibiotic resistance in a strain of bacteria. When a strain of bacteria is exposed to an antibiotic, the bacteria that are susceptible die. The more resistant bacteria may survive and multiply. If the bacteria are exposed to the antibiotic again, the selection process is repeated. With each exposure to the drug, the resistant bacteria gain a stronger foothold. Making matters worse, antibiotics kill beneficial bacteria along with the harmful ones. Normally, the beneficial bacterial strains help keep the harmful strains in check. Loss of the "good" bacteria can allow the harmful ones to dominate.

The overuse and misuse of antibiotics are largely to blame for the resistance problem. An example of overuse is when physicians prescribe antibiotics for illnesses that are viral, such as a cold or flu. This is overuse; antibiotics have no effect on viruses, so they are unnecessary for treating such illnesses. Patients misuse antibiotics when they stop taking their medicine as soon as they feel better instead of completing the full course of treatment. By stopping too early, they may be leaving the bacteria with greater resistance alive. Hospitals use antibiotics heavily, so it is not surprising that they are breeding grounds for antibiotic-resistant bacteria. The resistant bacteria survive, outgrow susceptible strains, and spread from person to person. Indeed, most infections by antibiotic-resistant bacteria occur in hospitals. An example is Staphylococcus aureus, which can cause many types of infections, including blood poisoning, pneumonia, skin infections, heart infections, and nervous system infections. The strain of S. aureus called MRSA (methicillin- resistant Staphylococcus aureus), is actually resistant to many antibiotics. For many years, MRSA existed only in hospitals, but it is now found in the community at large. For a time, vancomycin was the only antibiotic that remained effective against MRSA. Unfortunately, a vancomycin-resistant S. aureus (VRSA) has arisen. An antibiotic-resistant strain of another bacterium, Clostridium difficile (C. diff), is more dangerous than other strains, because it produces more toxin. Outbreaks of C. difficile are spreading in hospitals, because antibiotics are used heavily there. Hospital-acquired C. difficile infections cause 18,000 to 20,000 deaths a year in the United States.

More than 40% (by mass) of the antibiotics used in the United States are given to livestock to promote growth and ensure health. Farmers also spray crops with antibiotics to control or prevent bacterial infections in the crops. These practices also contribute to antibiotic resistance.

What can you do to slow the spread of drug-resistant bacteria? Use antibiotics responsibly. Do not insist on a prescription for antibiotics against your doctor's advice. Take antibiotics exactly as prescribed, and be sure to complete the recommended treatment. Also, reduce your risk of getting an infection that might require antibiotic treatment by washing your hands frequently, rinsing fruits and vegetables before eating them, and cooking meat thoroughly.

 

Viruses

Viruses are responsible for many human illnesses. Some viral diseases, such as the common cold, are usually not very serious. Other viral diseases, such as yellow fever, can be deadly.

Most biologists do not consider a virus to be a living organism because, on its own, it cannot perform any life processes (see Chapter 1 for a review on the basic characteristics of life). To copy itself, a virus must enter a host cell. The virus exploits the host cell's nutrients and metabolic machinery to make copies of itself that then infect other host cells.

Viruses are much smaller than bacteria. A virus consists of a strand or strands of genetic material, either DNA or RNA, surrounded by a coat of protein, called a capsid (Figure 13a.2). The genetic material carries the instructions for making new viral proteins. Some of these proteins become structural parts of the new viruses. Some of them serve as enzymes that help carry out biochemical functions important to the virus. Some are regulatory proteins, such as the proteins that trigger specific viral genes to become active under certain sets of conditions or the proteins that convert the host cell into a virus-producing factory.

 

 

FIGURE 13a.2. (a) The structure of a typical virus. A coat, called a capsid, made of protein surrounds a core of genetic information made of DNA or RNA. Some viruses have an outer membranous layer, called the envelope, from which glycoproteins project. (b) Steps in viral replication.

 

Which part of a virus would have to change for it to be able to infect a new type of tissue?

The glycoprotein on its surface. It is the fit between the glycoprotein and the host cell receptor that determines whether the virus can infect the cell.

 

Some viruses have an envelope, an outer membranous layer studded with glycoproteins. In some viruses, the envelope is actually a bit of plasma membrane from the previous host cell that became wrapped around the virus as it left the host cell. The envelope of certain other viruses—those in the herpes family, for instance—comes from a previous host cell's nuclear membrane. In any case, the virus produces the glycoproteins on the envelope. Some glycoproteins are important for attachment of the virus to the host cell.

A virus can replicate (make copies of itself) only when its genetic material is inside a host cell. Figure 13a.2 illustrates the general steps in the replication of viruses that infect animal cells:

1. Attachment. The virus gains entry by binding to a receptor (a protein or other molecule of a certain configuration) on the host cell surface. Such binding is possible because the viral surface has molecules of a specific shape (glycoproteins or capsids) that fit the host's receptors. The host cell receptors play a role in normal cell functioning. However, a molecule on the surface of the virus has a shape that is similar to the chemical that would normally bind to the receptor. Viruses generally attack only certain kinds of cells in certain species, because a particular virus can infect only cells bearing a receptor the virus can bind to. For example, the virus that causes the common cold infects only cells in the respiratory system, and the virus that causes hepatitis infects only liver cells.

2. Penetration. After a virus has bound to a receptor on a host cell, the entire virus enters the host cell, often by phagocytosis by the host cell. Once inside, the virus loses its capsid, leaving only its genetic material intact.

3. Production of viral genetic information and proteins. Viral genes then direct the host cell machinery to make thousands of copies of viral DNA or RNA. Next, viral genes direct the synthesis of viral proteins, including coat proteins and enzymes.

4. Assembly of new viruses. Copies of the viral DNA (or RNA) and viral proteins then assemble to form new viruses.

5. Release. Some viruses leave the cell through budding, or shedding. In this process, the newly formed viruses push through the host cell's plasma membrane and become wrapped in this membrane, which forms an envelope. Budding need not kill the host cell. Other virus types do not acquire an envelope, but rather cause the host cell membrane to rupture, releasing the newly formed viruses and killing the host cell.

Viruses can cause disease in several ways, as summarized in Table 13a.1. Some viruses cause disease when they kill the host cells or cause the cells to malfunction. The host cell dies when viruses leave it so rapidly that it lyses (bursts). In such cases, disease symptoms will depend on which cells are killed. However, if viruses are shed slowly, the host cell may remain alive and continue to produce new viruses. Slow shedding causes persistent infections that can last a long time. Some viruses can produce latent infections, in which the viral genes remain in the host cell for an extended period without harming the cell. At any time, however, the virus can begin replicating and cause cell death as new viruses are released.

 

TABLE 13a.1. Possible Effects of Animal Virus on Cells

 

 

An example of a virus that can act in all of these ways is the herpes simplex virus that causes fever blisters ("cold sores") on the mouth. The virus is spread by contact (discussed shortly) and enters the epithelial cells of the mouth, where it actively replicates. Rapid shedding kills the host cells, causing fever blisters. Slow shedding may not cause outward signs of infection, but the virus can still be transmitted. When the blisters are gone, the virus remains in a latent form within nerve cells without causing symptoms. However, stress can activate the virus. It then follows nerves to the skin and begins actively replicating, causing new blisters in the same region of the mouth.

Certain viruses can also cause cancer. Some do this when they insert themselves into the host chromosome near a cancer- causing gene and, in so doing, alter the functioning of that gene. Still other viruses bring cancer-causing genes with them into the host cell.

Unfortunately, viruses are not as easy to destroy as bacteria. One reason is the difficulty of attacking viruses inside their host cells without killing the host cell itself. Most attempts to develop antiviral drugs have failed for this reason. Nonetheless, some drugs are now available to slow viral growth, and others are being developed. Most of the antiviral drugs available today, including those against the herpes virus and HIV, work by blocking one of the steps necessary for viral replication. As mentioned in Chapter 13, interferons are proteins produced by virus-infected cells that protect neighboring cells from all strains of viruses. Interferons are not as useful as originally hoped, but they have been used for certain viral infections, including hepatitis C and the human papillomavirus that causes genital warts.

Because of these obstacles to treatment, the best way to fight viral infections is to prevent them with vaccines (discussed in Chapter 13).

 

Stop and think

How does the structure and replication cycle of viruses explain why antibiotics are not effective against viral diseases?

 

Protozoans

Protozoans are single-celled eukaryotic organisms with a well- defined nucleus. They can cause disease by producing toxins or by releasing enzymes that prevent host cells from functioning normally. Protozoans are responsible for many diseases, including malaria, sleeping sickness, amebic dysentery, and giardiasis. Giardiasis is a diarrheal disease that can last for weeks. There are frequent outbreaks of giardiasis in the United States, most of them resulting from water supplies contaminated with human or animal feces. Even clear and seemingly clean lakes and streams in the wilderness can contain Giardia (Figure 13a.3). Fortunately, drugs are available to treat protozoan infections. Some of these drugs work by preventing protozoans from synthesizing proteins.

 

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FIGURE 13a.3. Giardia is a protozoan that is commonly found in lakes and streams used as sources of drinking water, even those in pristine areas. It causes severe diarrhea that lasts for weeks and can be especially dangerous for children.

 

Fungi

Like the protozoans, fungi are also eukaryotic organisms with a well-defined nucleus in their cells. Some fungi exist as single cells. Others are organized into simple multicellular forms, with not much difference among the cells. There are more than 100,000 species of fungi, but fewer than 0.1% cause human ailments. Fungi obtain food by infiltrating the bodies of other organisms—dead or alive—secreting enzymes to digest the food, and absorbing the resulting nutrients. If the fungus is growing in or on a human, body cells of the human are digested, causing disease symptoms. Some fungi cause serious lung infections, such as histoplasmosis and coccidioidomycosis. Other, less-threatening fungal infections occur on the skin and include athlete's foot and ringworm. Most fungal infections can be cured. Fungal cell membranes have a slightly different composition from those of human cells. As a result, the membrane is a point of vulnerability. Some antifungal drugs work by altering the permeability of the fungal cell membrane. Others interfere with membrane synthesis by fungal cells. Fungal infections of the skin, hair, and nails can be combated with a drug that prevents the fungal cells from dividing.

 

Parasitic Worms

Parasitic worms are multicellular animals that benefit from a close, prolonged relationship with their hosts while harming, but usually not killing, their hosts. They include flukes, tapeworms, and roundworms, such as hookworms and pinworms. They can cause illness by releasing toxins into the bloodstream, feeding off blood, or competing for food with the host. Parasitic worms cause many serious human diseases, including ascariasis, schistosomiasis, and trichinosis.

Ascariasis is caused by a large roundworm, Ascaris, that is about the size of an earthworm. People become infected with Ascaris when they consume food or drink contaminated with Ascaris eggs. The eggs develop into larvae (immature worms) in the person's intestine. The larvae then penetrate the intestinal wall, enter the bloodstream, and travel to the lungs. After developing further, the worms are coughed up and swallowed, thus returning to the intestine. Within 2 to 3 months, they mature into male and female worms, which live for about 2 years. During those years, female worms can produce more than 200,000 eggs a day.

As much as 25% of the world population is infected with Ascaris, particularly in tropical regions. Up to 50% of the children in some parts of the United States (mostly rural areas in the Southeast) are infected. Many people with ascariasis have no symptoms. However, the worms can cause lung damage and severe malnutrition. When many worms are present, they can block or perforate the intestines, leading to death.

 

Prions

Prions (pree'-ons) are infectious particles of proteins—or, more simply, infectious proteins. They are misfolded versions of a harmless protein normally found on the surface of nerve cells. If a prion is present, it somehow causes the host protein to change its shape to the abnormal form. Prions cause a group of diseases called transmissible spongiform encephalopathies (TSEs), which are associated with degeneration of the brain. The misshapen proteins clump together and accumulate in the nerve tissue of the brain. These clumps of prions may damage the plasma membrane or interfere with molecular traffic. Spongelike holes develop in the brain, causing death.

Transmissible spongiform encephalopathies are progressive and fatal. Prions cannot be destroyed by heat, ultraviolet light, or most chemical agents. Currently, there is no treatment for any disease they cause. Several of the TSEs are animal infections, notably mad cow disease, scrapie in sheep, and chronic wasting disease (CWD), which affects deer and elk.

Prions also cause a human neurological disorder called Creutzfeldt-Jakob disease (CJD). Indeed, the prion responsible for mad cow disease is thought to cause one form of CJD. The incubation period for CJD can be months to decades. Symptoms include sensory and psychiatric problems. Once the symptoms begin, death usually occurs within a year.

How does an animal become infected with prions? In the case of mad cow disease, it appears that cattle become infected when they eat prions in contaminated food. For example, prions have been passed along in the protein supplements fed to cattle to increase their growth and milk production. Those protein supplements had been prepared from the carcasses of animals considered unfit for human consumption, a practice banned in the United States in 1997. A broader ruling prohibiting the use of any high-risk animal parts in any animal feed went into effect in 2009. Any protein supplements prepared from animals infected with mad cow disease would have contained prions. The prions pass through the intestinal wall, enter the lymphatic system, and are then transported by nerves to the brain and spinal cord. In contrast, CWD can apparently be spread by animal-to-animal contact, including contact with body fluids such as urine or feces from infected animals. Scientists also think that the prions responsible for CWD may remain in the soil or water for years. As a result, healthy animals may become infected from living in a region previously occupied by diseased animals. Humans can become infected with prions by eating contaminated substances, through tissue transplant, or through contaminated surgical instruments.

 

What would you do?

Mad cow disease is spread in cattle when they consume contaminated food. There are laws to prevent feeding cattle food that might be contaminated. When violations of the laws occur, who should be held responsible: the food manufacturers or the farmers?

 

Spread of a Disease

 

Obviously, you catch a disease when the pathogen enters your body. But how do diseases travel from person to person or enter the body in the first place? The answer to this question varies with the type of pathogen.

• Direct contact. One means of transmission is direct contact of an infected person with an uninfected person, as might occur when shaking hands, hugging and kissing, or being sexually intimate. For example, sexually transmitted diseases (STDs) including chlamydia, gonorrhea, syphilis, genital herpes, and HPV are spread when a susceptible body surface touches an infected body surface (see Chapter 17a). The organisms that cause STDs generally cannot remain alive outside the body for very long, so direct intimate contact is necessary. A few disease-causing organisms—HIV and the bacterium that causes syphilis, for instance—can spread across the placenta from a pregnant woman to her growing fetus.

• Indirect contact. Indirect contact, the transfer from one person to another without their touching, can spread other diseases. Most respiratory infections, including the common cold, are spread by indirect contact (see Chapter 14). When an infected person coughs or sneezes, airborne droplets of moisture full of pathogens are carried through the air (Figure 13a.4). The infected droplets may be inhaled or land on nearby surfaces. When another person touches an affected surface, the organisms are transmitted. In this way, droplet infection spreads pathogens on contaminated inanimate objects, including doorknobs, drinking glasses, and eating utensils.

 

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FIGURE 13a.4. Pathogens can be spread through the air in droplets of moisture when an infected person sneezes or coughs.

 

• Contaminated food or water. Certain diseases are transmitted in contaminated food or water. You have read that spoiled food can cause food poisoning. Another disease transmitted by food or water is hepatitis A, an inflammation of the liver caused by a certain virus. Legionella, the bacterium that causes a severe respiratory infection known as Legionnaires' disease, is a common inhabitant of the water in condensers of large air conditioners and cooling towers. The disease-causing bacteria are spread through tiny airborne water droplets. Coliform bacteria come from the intestines of humans and are, therefore, an indicator of fecal contamination of water. Their numbers are monitored in drinking and swimming water. To be safe, drinking water should not have any coliform bacteria.

• Animal vectors. Another means of transmission is by vector, an animal that carries a disease from one host to another. The most common vector-borne disease in the United States is Lyme disease. It is caused by a bacterium transmitted by the deer tick (the vector), which is about the size of the head of a pin (Figure 13a.5). The tick larva picks up the infectious agent when it bites and sucks blood from an infected animal. When the tick subsequently feeds on a human or other mammalian host, the bacteria gradually move from the tick's gut to its salivary glands and then are passed to its victim. The incubation period, during which there are no symptoms, can be as long as 6 to 8 weeks. Early symptoms include a headache, backache, chills, and fever. Often, a rash resembling a bull's eye develops, with an intense red center and border. Over a period of weeks, the circle increases in diameter. Weeks to months later, unless the disease is treated promptly, pain, swelling, and arthritis may develop. Cardiovascular and nervous system problems may follow the arthritis.

 

 

FIGURE 13a.5. A tiny tick, the deer tick, is a vector that transmits the bacterium responsible for Lyme disease. One characteristic sign of Lyme disease is a red bull’s-eye rash surrounding the tick bite. The rash gradually increases in diameter.

 

Mosquitoes transmit the West Nile virus, which can cause both meningitis (inflammation of the meninges, the protective coverings of the central nervous system) and encephalitis (brain inflammation; Figure 13a.6).

 

 

FIGURE 13a.6. Mosquitoes are the vector that transmits West Nile virus.

 

The first reported cases of West Nile virus in North America were in New York City in 1999. Since then, the disease has spread to nearly every state. The virus can infect certain vertebrates, including humans, horses, birds, and occasionally dogs and cats. Testing mosquitoes and dead birds, especially crows and starlings, for the virus is one way to track its spread. Because the symptoms are similar to those of the flu—fever, headache, and muscle and joint pain—many people who become infected are unaware of it—and most infected people under the age of 50 have few symptoms or none at all. However, older people have weaker immune systems. If they become infected, they are more likely to develop meningitis or encephalitis, either of which can cause brain damage, paralysis, or death.

You can protect yourself from West Nile virus and other mosquito-borne viral infections such as Eastern equine encephalitis by avoiding wet and humid places that harbor mosquitoes. If you must enter an area where mosquitoes are likely to be, wear light-colored clothing that covers your body, and use insect repellent.

 

Infectious Diseases as a Continued Threat

 

An epidemic is a large-scale outbreak of an infectious disease. The most notorious epidemics—bubonic plague, cholera, diphtheria, and smallpox—have happened in the distant past, although new outbreaks may occur sporadically. However, outbreaks of serious new diseases continue to present problems. We discuss some of these modern-day plagues elsewhere in the text.

 

Emerging Diseases and Reemerging Diseases

An emerging disease is a disease with clinically distinct symptoms whose incidence has increased, particularly over the last two decades. Among these diseases are HIV, SARS, H1H5 influenza, and H1N1 influenza. Other diseases have reemerged that were thought to have been conquered. A reemerging disease is a disease that has reappeared after a decline in incidence. For example, due to new drug-resistant strains of bacteria, tuberculosis is once again a global problem. We consider three factors that play important roles in the emergence and reemergence of disease.

1. Development of new organisms that can infect humans and of drug-resistant organisms. Most of the time, a pathogen infects only one type or a few types of organisms. Mutations are changes in genetic information that occur randomly. Some mutations allow the pathogen to "jump species" from its original host and infect another type of organism. Recall that a virus can penetrate a cell only if the virus has the appropriate molecule on its surface—one that will fit into a receptor on the host cell. Another mechanism that could allow an animal virus to infect humans is mixing of genetic information of an animal virus and a human virus, which might occur if both viruses infected the same cell. This is how the H1N1 virus that causes swine flu developed. A person passed human influenza A viruses to a pig with influenza A. When the viruses infect the same cell, pieces of the viruses' genetic material get mixed and create a new strain of virus.

Pathogens can also undergo changes in their response to drugs. We have seen that certain bacteria have acquired resistance to antibiotics, for example. As a result, some diseases that were once easily cured by antibiotics are now much more difficult to treat.

Improper antibiotic treatment during the reemergence of tuberculosis (TB) has led to antibiotic-resistant strains of TB that, in turn, make TB more difficult to treat. Infections with multi-drug-resistant strains of Mycobacterium tuberculosis, the bacterium that causes TB, are increasing at an alarming rate. The World Health Organization coined a new term to describe drug resistance in a new strain of the tuberculosis bacterium—XDR, which stands for extensively drug resistant. The new XDR strain causes a tuberculosis infection that is nearly impossible to treat.

2. Environmental change. Changes in local climate—the annual amount of rainfall and the average temperature—can affect the distribution of organisms and change the size of the geographical region where certain organisms can live. Global warming makes the redistribution of pathogens a growing concern.

3. Population growth. Another important factor in the emergence or reemergence of diseases is the increase of the human population in association with the development and growth of cities. Swelling human populations in cities cause people to move out of the city into surrounding areas, creating suburbs. If the surrounding areas were previously undeveloped, the move brings more people into contact with animals and insects that might carry infectious organisms. Indeed, wild animals serve as reservoirs for more than a hundred species of pathogens that can affect humans. The development of suburbs also destroys populations of predators, such as foxes and bobcats. In some regions of New York, the loss of predators has led to an increase in numbers of tick-carrying mice and an increase in the incidence of Lyme disease.

Population density and mobility also enable infectious diseases to spread more easily today than in the past. Densely populated cities allow diseases to begin spreading quickly, and air travel enables them to spread over great distances, (Figure 13a.7).

 

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FIGURE 13a.7. Air travel is one reason that new diseases can spread rapidly.

 

Global Trends in Emerging Infectious Diseases

Emerging infectious diseases are a concern because of economic costs and public health issues. These diseases are not evenly distributed throughout the world. The most important factors determining where new infectious disease will emerge are (1) the rate of human population growth and the density of the human population, and (2) the number of species of wild mammals. Most pathogens responsible for emerging infectious diseases are spread to humans by animals—wildlife, pets and livestock, and vectors. As we have seen, the development of drug resistance in some pathogens has also led to emergent infectious diseases.

 

Epidemiology

Epidemiology is the study of patterns of disease, including rate of occurrence, distribution, and control. Most diseases can be described as having one of the following four patterns:

• Sporadic diseases occur only occasionally at unpredictable intervals. They affect a few people within a restricted area.

• Endemic diseases are always present in a population and pose little threat. The common cold provides an example.

• An epidemic disease occurs suddenly and spreads rapidly to many people. Outbreaks of smallpox and cholera are examples of epidemics.

• A pandemic is a global outbreak of disease. HIV/AIDS is considered to be a pandemic.

Epidemiologists are "disease detectives" who try to determine why a disease is triggered at a particular time and place. The first step in answering this question is to verify that there is indeed a disease outbreak, defined as more than the expected number of cases of individuals with similar symptoms in a given area. Next, epidemiologists try to identify the cause of the disease; whether it can be transmitted to other people; and, if it can be, how the disease is transmitted. To identify the cause of an infectious disease, epidemiologists try to isolate the same infectious agent from all people showing symptoms of the condition. They also try to identify factors—including age, sex, race, personal habits, and geographic location—shared by people with symptoms of the condition. These factors might provide a clue as to whether the condition can be transmitted and how.

 

Looking ahead

In this chapter, we considered infectious disease—the pathogens that cause them, the methods by which they spread, reasons for emerging and reemerging diseases, and the epidemiologists that track the causes. In the next chapter we examine the respiratory system, which brings life-giving oxygen into the body and rids the body of carbon dioxide.

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Bacteria resistant to all antibiotics available today include some strains of Staphylococcus aureus (skin infection, pneumonia), Mycobacterium tuberculosis (tuberculosis), Enterococcus faecalis (intestinal infections), and Pseudomonas aeruginosa (many types of infections).