II
Disease Threats

Our “war” on infectious microbes has restricted the spread of several pathogens and drastically reduced the burden of human disease, but we are a long way from conquering infectious diseases.


During the past three decades, 37 new human pathogens have been identified as disease threats. An estimated 12 percent of known human pathogens have been recognized as either emerging or re-emerging. Having fallen steadily since the turn of the 20th century, the number of deaths attributable to infection in the United States began to increase in the early 1980s due, in large part, to the HIV/AIDS pandemic and increases in deaths attributable to pneumonia and bloodstream infections. Infectious disease continues to cause high morbidity and mortality throughout the world, particularly in developing countries.


It accounts for about a quarter of deaths worldwide, and in 2008 caused more than two-thirds of the estimated 8.8 million deaths in children under the age of five. What are some of the most significant microbial threats we face?

Animal Carriers

Many of the diseases that afflict people today are caused by microbes whose ancestors came from animals first domesticated by early humans. Biologists believe that the measles virus stemmed from canine distemper and rinderpest, an affliction of cattle; that rhinoviruses, agents of the common cold, came to us from horses; and that smallpox is a close cousin of cowpox.


Infections transmitted from animals to humans are called zoonoses, or zoonotic diseases. Of the more than 1,700 known viruses, bacteria, and other pathogens that infect people, more than half either originated in or now come directly from animals;



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II Disease Threats Our “war” on infectious It accounts for about a quarter of deaths worldwide, and in 2008 microbes has restricted caused more than two-thirds of the the spread of several estimated 8.8 million deaths in children under the age of five. What are some of the pathogens and drastically most significant microbial threats we face? reduced the burden of human disease, but we are a long way from Animal Carriers conquering infectious diseases. Many of the diseases that afflict people today During the past three decades, 37 new human are caused by microbes whose ancestors came pathogens have been identified as disease threats. from animals first domesticated by early humans. An estimated 12 percent of known human Biologists believe that the measles virus stemmed pathogens have been recognized as either emerging from canine distemper and rinderpest, an affliction or re-emerging. Having fallen steadily since the of cattle; that rhinoviruses, agents of the common turn of the 20th century, the number of deaths cold, came to us from horses; and that smallpox is a attributable to infection in the United States close cousin of cowpox. began to increase in the early 1980s due, in large part, to the HIV/AIDS pandemic and increases in Infections transmitted from animals to humans are deaths attributable to pneumonia and bloodstream called zoonoses, or zoonotic diseases. Of the more infections. Infectious disease continues to cause than 1,700 known viruses, bacteria, and other high morbidity and mortality throughout the pathogens that infect people, more than half either world, particularly in developing countries. originated in or now come directly from animals; 11

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are used for food and medicine in many parts of Asia, and was then “amplified” through the infection of civet cats, a step leading to a mutation that makes the disease transmissible to humans. The virus infected 8,098 people, of which 774 died—a nearly 10 percent mortality rate. Fortunately, no human infections have been found since early 2004. There are many other examples of direct transmission. Toxoplasmosis, a parasitic disease that typically causes mild flu-like symptoms in humans (but potentially more serious illness in individuals with compromised immune systems), infects many Animals domesticated by early humans are believed to have been the warmblooded animals. Cats play an important role source of microbes that eventually caused disease in people, as in the in spreading the disease when they become infected case of measles stemming from canine distemper and rinderpest. by eating infected rodents or small birds and then pass the parasite to humans through their feces. Leptospirosis, a bacterial disease the rest come from the environment around us, such spread through the urine as soil, water, and air. And of the 37 new infectious of infected animals, or diseases identified in the past 30 years, more than through soil or water two-thirds sprang from animals. The next deadly contaminated by pandemic to sweep the world could very likely jump infected urine, can species in this way. cause a wide range of symptoms in Direct Transmission humans, including Some zoonotic infections move directly from high fever, vomiting, animals to humans. In such cases, an animal is the and even meningitis natural host—or reservoir—for the pathogen, and and liver failure. The through an evolutionary twist of fate, the pathogen Nipah virus, which can moves from the natural host to humans. Severe cause fatal encephalitis Acute Respiratory Syndrome (SARS) is a recent (inflammation of the brain), example of this. In the spring of 2003 this new Fruit bat, which can emerged in 1998 in Malaysia. and deadly viral illness swept out from China’s transmit the Nipah virus. Harbored in fruit bats, the virus Guangdong Province and spread rapidly around the afflicted slaughterhouse workers world before it was contained that summer. SARS who had caught it from pigs. originated in Chinese horseshoe bats, animals that 12

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Indirect Transmission Diseases that are transmitted to humans indirectly via an insect, an arthropod (animals with jointed appendages and exoskeletons, such as ticks), or another animal (such as snails, which deliver the parasite responsible for schistosomiasis) are called vector-borne diseases. Vectors carry disease- causing viruses, bacteria, or parasites from one host to another, delivering these pathogens to humans and other warmblooded hosts. The vectors themselves typically suffer no ill effects from the organisms they carry. American dog tick, potential vector of Rocky In 1999, for example, a Mountain spotted fever. mosquito-borne infection— A vector ecologist tests crows for the West Nile virus in a lab in St. Paul, West Nile virus—suddenly Minnesota. Dead crows were one of the early signs of the virus’s began targeting New Yorkers. Seven people died and presence in North America. 62 were hospitalized. Until then the virus had been confined to Africa, West Asia, and the Middle East. City strain of the virus was virtually identical to a Today, the infection caused by West Nile virus has strain taken the previous year from a dead goose in fully established itself in North America, flaring up in Israel. Scientists speculate that an infected mosquito, the summer and continuing into the fall. Since 1999 human, or bird may have brought the pathogen to the virus has spread rapidly across North America and this country on a plane or ship. into Latin America. In 2009 there were 720 reported cases of West Nile virus in the United States, of which Many other common infections, including malaria, 32 were fatal. yellow fever, Lyme disease, and typhus, are spread to humans from animals via the bites of insects and other Wild or domestic animals are natural reservoirs for arthropods. In fact, nearly half the world’s population many vector-borne diseases. The main reservoir is currently infected with a vector-borne disease. host for West Nile virus is wild birds. The New York 13

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Foodborne Pathogens Each year an estimated 76 million Americans— about one in four—become infected by what they eat. Approximately 325,000 are hospitalized. More than 5,000 (14 a day) die. In April 2009 the U.S. Centers for Disease Control and Prevention (CDC) reported that progress in reducing foodborne infections had stalled, pointing to gaps in the existing food safety system and the need to develop improved food safety practices as products move from the farm to the table. The true magnitude of foodborne illness is likely to be much higher than even the official estimates because most people do not seek medical attention for its symptoms, such as abdominal cramps, vomiting, and diarrhea. The Pathogens Behind Foodborne Illness Foodborne disease occurs when a susceptible host consumes contaminated foods or beverages. Many different disease-causing microorganisms—bacteria, viruses, and parasites—can taint foods and liquids, each potentially associated with a different illness. The most common causes of foodborne illness Salad bars may harbor agents of foodborne illness, such as Salmonella include the bacterial infections Campylobacter, bacteria (below), if the food has not been handled properly. the most frequently identified bacterial cause of diarrheal illness in the world; Salmonella, which spreads to humans through a variety of foods of animal origin, or through fecal contamination of plant-based foods, such as in the 2009 peanut- product outbreak; and E. coli O157:H7, the agent behind a serious and sometimes deadly complication called hemolytic-uremic syndrome (HUS). The most common viral cause of foodborne 14

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illness is Calicivirus, also referred to as Norwalk- like virus or norovirus. Unlike the previous three bacterial foodborne pathogens, noroviruses easily spread from one infected person to another and can contaminate an environment, making them extremely difficult to eradicate from hotels, hospitals, nursing homes, cruise ships, and similar establishments where large numbers of people congregate. After you swallow a foodborne pathogen there may be a delay—the incubation period—before symptoms appear. This delay may range from hours to days. During the incubation period, the microbes pass through the stomach into the intestine, attach to the cells lining the intestinal walls, and begin to multiply there. Some types of microbes stay in Outbreaks of norovirus (Calicivirus), the most common form of viral foodborne illness, spread easily in places where large numbers of the intestine; some, like cholera, produce a toxin people are gathered together, such as cruise ships. that causes the body to secrete water, resulting in diarrhea. Others, like the typhoid bacillus, invade Mangoes are sliced in preparation for exportation from South America. and replicate in the deeper body tissues. Global food distribution increases the risk of widespread epidemics if food becomes contaminated. Not all foodborne pathogens require an incubation period, however. Illness can result from toxins that form in the food before it is eaten—leading to true “food poisoning.” In such cases, bacteria do not need to replicate in the body at all and the onset of symptoms can be more rapid. What Causes Outbreaks? In the past few decades, food production and distribution for the developed world have increasingly involved vast and intricate global networks. This sprawling system produces food that, if contaminated, increases the potential for widespread epidemics. In this giant food economy 15

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opportunities abound for Foods pooled from many sources, such as batches of raw ground beef, food to come in contact can become tainted if any of the meat with pathogens. Meat and in the batch is contaminated with a poultry carcasses can become human pathogen. contaminated during slaughter by contact with small amounts of intestinal contents. Fresh fruits and vegetables become tainted be cooked to an internal temperature of 160oF. Eggs if they are washed or irrigated with water contaminated with should be cooked until the animal manure or human yolk is firm. sewage. (Outbreaks related Separate: Avoid cross-contaminating foods by to fresh produce have increased eightfold in the washing hands, utensils, and cutting boards after United States during the past several decades.) And contact with raw meat or poultry and before they increasingly, we don’t cook our own meals, leaving touch another food. Unless it is disinfected between food safety in the hands, literally, of others. each use, don’t use a “universal” cleanup tool such as a sponge. Place cooked meat on a clean platter, Raw foods of animal origin are the most likely to be rather than back on the one that held the raw meat. contaminated—that is, raw meat and poultry, raw Chill: Bacteria can grow quickly at room eggs, unpasteurized milk, and raw shellfish. Foods temperature, so refrigerate leftover foods if they are for which such products are pooled from many not going to be eaten within 4 hours. A large volume sources and batch processed are also hazardous, of food will cool more quickly if divided into several because a pathogen present in any one of the shallow containers for refrigeration. animals might contaminate the whole batch. Clean: Rinse fresh fruits and vegetables in running How to Protect Yourself tap water to remove visible dirt and grime. Remove Consumers can reduce the risk of foodborne illness and discard the outermost leaves from a head of by adhering to the following safe food handling and lettuce or cabbage. Because bacteria can grow on preparation practices: the cut surface of fruits or vegetables, be careful not to contaminate these foods while slicing them on Wash hands thoroughly with soap and warm water a cutting board, and avoid leaving cut produce at before handling food. room temperature for many hours. Cook meat, poultry, and eggs thoroughly. Use a Report suspected foodborne illness to your local thermometer to measure the internal temperature health department. of meat, to be sure that it is cooked sufficiently to kill bacteria. Ground beef, for example, should 16

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one-fifth of all deaths of children under the age of Global Killers five. These infections are so widespread in developing countries that parents often fail to recognize when A handful of deadly infectious diseases claim millions symptoms become critical. Children die simply of lives worldwide each year: lower respiratory because their bodies are weakened—often through tract infections, diarrheal diseases, HIV/AIDS, rapid loss of fluids and undernourishment. The tuberculosis, and malaria. Together, they account burden of diarrheal diseases is highest in deprived for nearly one-fifth of deaths globally. Several of areas where there is poor sanitation, inadequate these diseases have plagued humankind throughout hygiene, and unsafe drinking water. history, often decimating populations with greater efficiency than wars. In an age of vaccines, HIV stands for human immunodeficiency virus, antibiotics, and dramatic scientific progress, these the virus that causes acquired immunodeficiency diseases should have been brought under control. Yet syndrome (AIDS), the final stage of HIV infection. they continue to kill at an alarming rate, particularly HIV appears to have jumped to humans early in the in the developing world. In low-income countries 20th century from a type of chimpanzee in West the dominant causes of death are infectious and Africa—most likely when humans hunted these parasitic diseases (including malaria) and poor animals for meat and came into contact with their medical care surrounding childbirth. By contrast, infected blood. The virus slowly spread across Africa in high-income countries the leading causes of and later to other parts of the world. death are noncommunicable diseases, such as heart disease and cancer. Infectious and parasitic causes of In 2008 more than 33 million people worldwide mortality are farther down on the list. were infected with HIV and an estimated 2 million died. Unlike most other viruses, HIV attacks the Lower respiratory tract infections (including pneumonia) account for more than 4 million deaths worldwide—the greatest global killer among infectious diseases. Pneumonia is also the leading cause of death of the very young, often striking children with low birth weight or those whose immune systems are weakened by malnutrition or other diseases. Most of these deaths occur in developing countries. Diarrheal diseases are the second-leading cause of infectious disease deaths worldwide, accounting for more than 2 million deaths each year, and nearly HIV particles (green spheres). 17

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immune system, destroying a type of white blood to the standard first-line drugs that are most easily cell (T cells or CD4 cells) that the immune system tolerated and have historically worked best against needs to fight disease. HIV is transmitted by having the disease. Extensively drug-resistant TB (XDR-TB) sex with someone infected with HIV, by sharing occurs when resistance to second-line drugs, which needles and syringes with an infected person, are typically more toxic and less effective, develops through blood or blood product transfusions, or by on top of MDR-TB. Such infections are highly lethal. being exposed as a fetus or an infant to the virus before or during birth or through breastfeeding. HIV On the global stage, HIV/AIDS and TB are tragically is not transmitted through casual contact, such as interconnected. Among HIV-infected people whose shaking hands, hugging, modest kissing, or drinking immune systems are weakened by the effects of the from the same glass. virus, the risk of developing TB is much higher than for persons with normal immune systems. Indeed, Someone in the world is newly infected with the TB is the leading cause of death worldwide among tuberculosis (TB) bacterium every second. In 2008 people infected with HIV. Likewise, among people there were an estimated 9.4 million new cases with latent (inactive) TB infection, HIV infection is of tuberculosis and 1.8 million deaths. The vast the strongest known risk factor for progressing to majority of TB deaths are in the developing world, active TB disease. and more than half of all deaths occur in Asia. In the United States, 12,904 TB cases were reported in 2008, a nearly 3 percent decline from the number reported in 2007. TB, which is caused by the bacterium Mycobacterium tuberculosis, is a contagious disease that spreads through the air when an infected person coughs or sneezes and people nearby breathe in the bacteria. TB bacteria can live in the body without making an individual sick, a condition known as latent TB infection. More than 2 billion people—about one- third of the world’s population—are infected, many asymptomatically, with TB bacilli. A person with active TB can be treated by taking several drugs for 6 to 12 months. Multidrug-resistant TB (MDR-TB) is a new threat. It Crowded housing with poor sanitation may enable the rapid spread of is difficult and expensive to treat and fails to respond disease within a community. 18

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Like the other global killers, malaria occurs mostly in poor, tropical, and subtropical areas of the world. Each year as many as 300 million people are infected with malaria worldwide, and up to 1 million die, most of them young children in sub- Saharan Africa. Malaria is a mosquito-borne disease caused by several different protozoan parasites. Humans infected with malaria parasites, depending on the type, can develop a wide range of illnesses, from mild infection that does not produce symptoms to the classic symptoms of malaria (fever, chills, sweating, headaches, muscle pains) to severe Villagers in sub-Saharan Africa learn how to set up a mosquito bednet complications (anemia, kidney failure, coma) that for protection against malaria. can lead to death. Malaria serves as a particularly dramatic example water development projects, such as dams, which of infectious disease re-emergence and illustrates create new mosquito breeding sites. the challenges of controlling human vector-borne diseases. Following the drastic depopulation Yet despite today’s dire headlines, progress is of Anopheles mosquitoes in the first half of the being made against the world’s modern infectious 20th century due to liberal use of the pesticide scourges. The scale-up of antiretroviral therapy has DDT, malaria began its resurgence in Asia in the reduced the number of AIDS deaths and mother- late 1960s. In Sri Lanka, where only 17 cases of to-child transmissions, and has improved survival malaria were reported in 1963, a major epidemic and productivity. Wider access to antiretroviral of more than 440,000 cases erupted 5 years later, treatments has also been accompanied by a after preventive vector control was replaced with dramatic reduction in prices. The prevalence of TB a strategy of identifying and treating new cases of has declined since 2000, partly because the WHO’s the illness. Similarly, by the mid-1970s millions of Directly Observed Therapy Short-Course strategy new cases had occurred in India after mosquito brought treatment and a cure to tens of millions of eradication efforts ceased. In Africa, where vector patients. And malaria deaths have fallen with the control programs were never initiated, a more recent development of artemisinin-based drugs, distribution upsurge in infections, including several explosive of insecticide-treated bed nets, and indoor residual epidemics, has erupted in endemic areas. A number spraying of insecticides. Alongside these efforts of factors appear to be driving this global resurgence, there have been major investments in health care including the rapid spread of drug resistance among systems—bolstering infrastructure, laboratories, and malarial parasites, changing rainfall patterns, and human resources. 19

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Influenza and countertops, for 2 to 8 hours—one of the reasons that hand washing and surface hygiene is an important part of most flu control strategies. The “flu” has become a popular catch-all term to describe anything from a bad cold to stomach Once the flu virus makes contact with mucous distress. But the real flu, influenza, is a defined membranes in the eyes and nose, it heads to the cells illness that many public health officials dread most. along the upper respiratory tract, bronchial tubes, and Each year in the United States about 36,000 people trachea, where it swiftly multiplies. Scientists believe flu die from flu-related complications, and more than symptoms arise because growth of the virus damages 200,000 are hospitalized. the cells into which it has inserted itself and because the immune system, in trying to limit the damage, Of even greater concern is a flu pandemic—a responds in ways that cause familiar discomfort: It worldwide epidemic of a new strain of influenza sends out white blood cells that release chemicals virus from which the human called cytokines, causing muscle and joint pain, and population has no immunity. it produces a fever, which is one of the body’s ways of Depending on its severity, an mobilizing its defenses against invaders. influenza pandemic could result in 200,000 to 2 million Seasonal Versus Pandemic Flu deaths in the United States The genius of the influenza virus lies in its ability alone. In 2009 the WHO to alter itself. The virus uses RNA rather than DNA declared the current H1N1 as its genetic material. RNA viruses make frequent “swine flu” a pandemic, mistakes while copying themselves. Their high although its human mortality mutation rate means that RNA viruses evolve far rate to date has been relatively more rapidly than DNA viruses, because every modest, with about 18,000 successive generation is a little different from the 2009 H1N1 influenza virus. confirmed deaths worldwide. previous one. The flu virus’s surface proteins— hemagglutinin (H) and neuraminidase (N)—are also How the Flu Spreads changeable. These proteins have a role in making Influenza viruses mainly spread when droplets it possible for a virus to invade and hijack cells. from the cough or sneeze of an infected person are Hemagglutinin permits virus particles to gain access propelled through the air and land on the mouth or to the cell’s interior, and neuraminidase helps newly nose of someone nearby. Flu viruses may also spread produced copies of the virus break free of the cell in when a person touches respiratory droplets on another quest of other cells to invade. person or on an object and then touches his or her own mouth or nose. The hardy influenza virus can There are three types of influenza viruses: A, B, survive on environmental surfaces, such as doorknobs and C. Only influenza A viruses are further 20

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classified by subtype on the basis of the H and Sometimes the virus’s surface proteins undergo N surface proteins. Influenza A subtypes and B a radical change—a process known as antigenic viruses are further classified by strains. Among shift—resulting in an altogether new influenza influenza A viruses, there are 16 known subtypes subtype against which most humans have no of hemagglutinin and 9 of neuraminidase. Many immunity. The result can be a pandemic. The extent different combinations of these H and N proteins are and severity of a pandemic depend on the specific possible, each representing a different subtype. characteristics of the virus. While rare, pandemics sweep the world like wildfire. In addition to the According to the CDC, the subtypes of influenza that recent H1N1 pandemic, three major pandemics are currently circulating among people worldwide broke out in the 20th century: an H1N1 in 1918 include A H1N1, A H3N2, and B strains. Usually (the misnamed “Spanish” flu), an H2N2 in 1957 only one subtype predominates in a given flu season. (the “Asian” flu), and an H3N2 in 1968 (the “Hong Epidemics break out every year because of slight Kong” flu). Of these pandemics, the 1918–1919 genetic mutations in a virus subtype’s surface proteins virus was the most fearsome, killing 50 million that result in a new strain of the virus—a process to 100 million people worldwide (or between known as antigenic drift. New combination vaccines 0.5 and 1 percent of the global population at are formulated annually to protect against the three that time). Many of those deaths were due to the circulating strains of seasonal flu that experts predict effects of pneumococcal pneumonia, a secondary will cause the most illness in the coming season. complication of flu for which no antibiotics During the 1918 flu pandemic, emergency hospitals were set up quickly for the huge influx of patients (left). A CDC microbiologist works carefully with a recreated 1918 virus to identify the characteristics that made this organism such a deadly pathogen (below). 21

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Influenza and countertops, for 2 to 8 hours—one of the reasons that hand washing and surface hygiene is an important part of most flu control strategies. The “flu” has become a popular catch-all term to describe anything from a bad cold to stomach Once the flu virus makes contact with mucous distress. But the real flu, influenza, is a defined membranes in the eyes and nose, it heads to the cells illness that many public health officials dread most. along the upper respiratory tract, bronchial tubes, and Each year in the United States about 36,000 people trachea, where it swiftly multiplies. Scientists believe flu die from flu-related complications, and more than symptoms arise because growth of the virus damages 200,000 are hospitalized. the cells into which it has inserted itself and because the immune system, in trying to limit the damage, Of even greater concern is a flu pandemic—a responds in ways that cause familiar discomfort: It worldwide epidemic of a new strain of influenza sends out white blood cells that release chemicals virus from which the human called cytokines, causing muscle and joint pain, and population has no immunity. it produces a fever, which is one of the body’s ways of Depending on its severity, an mobilizing its defenses against invaders. influenza pandemic could result in 200,000 to 2 million Seasonal Versus Pandemic Flu deaths in the United States The genius of the influenza virus lies in its ability alone. In 2009 the WHO to alter itself. The virus uses RNA rather than DNA declared the current H1N1 as its genetic material. RNA viruses make frequent “swine flu” a pandemic, mistakes while copying themselves. Their high although its human mortality mutation rate means that RNA viruses evolve far rate to date has been relatively more rapidly than DNA viruses, because every modest, with about 18,000 successive generation is a little different from the 2009 H1N1 influenza virus. confirmed deaths worldwide. previous one. The flu virus’s surface proteins— hemagglutinin (H) and neuraminidase (N)—are also How the Flu Spreads changeable. These proteins have a role in making Influenza viruses mainly spread when droplets it possible for a virus to invade and hijack cells. from the cough or sneeze of an infected person are Hemagglutinin permits virus particles to gain access propelled through the air and land on the mouth or to the cell’s interior, and neuraminidase helps newly nose of someone nearby. Flu viruses may also spread produced copies of the virus break free of the cell in when a person touches respiratory droplets on another quest of other cells to invade. person or on an object and then touches his or her own mouth or nose. The hardy influenza virus can There are three types of influenza viruses: A, B, survive on environmental surfaces, such as doorknobs and C. Only influenza A viruses are further 20

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provide hopeful signs. Scientists are also exploring ways to make a vaccine that is effective against all flu strains—a so-called universal vaccine. Such a development could dramatically improve the public’s protection against influenza infection. How to Protect Yourself • Consider getting vaccinated against influenza. Vaccines are one of the best ways to reduce the Alcohol-based hand sanitizers are effective in preventing the morbidity and mortality associated with the disease. transmission of many pathogens. The Food and Drug Administration They do not themselves cause influenza in any form. recommends a concentration of at least 60 percent ethanol. In healthy households, the use of other antibacterial agents, such as triclosan • Cover your nose and mouth with a tissue when (common in many soaps and detergents), may contribute to the problem you cough or sneeze. Throw the tissue in the trash of antibiotic resistance and should be used prudently. after you use it. • Wash your hands often with soap and water, • Avoid touching your eyes, nose, or mouth, which especially after coughing or sneezing. Alcohol-based can spread germs. hand sanitizers are also effective. • If you do get sick, stay home from work or school and limit your contact with others to keep from Injecting eggs with the flu virus to develop vaccines. infecting them. • Ask your doctor whether you should take an anti-influenza drug, such as Tamiflu, which can be effective if taken within 48 hours of developing the symptoms of flu. Antibiotic Resistance Antibiotics—medicine’s “magic bullets”—save tens of thousands of lives annually in the United States. But these magic bullets are losing their power. The problem is growing antibiotic resistance—the ability of bacteria to resist the effects of an antibiotic. Antibiotic resistance occurs when bacteria undergo a genetic change that reduces or eliminates the 23

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effectiveness of drugs or other agents units. Resistant pathogens lead to designed to cure or prevent infection. higher health care costs because they Resistant bacterial infections have often require more expensive drugs inevitably followed the widespread and extended hospital stays. use of every new antibiotic introduced. But healthy people are also at risk. A child with an ear infection that in The more we use antibiotics, the the early 1990s would have been more widespread bacterial resistance to these drugs instantly cured by penicillin may now need two, becomes. Every time a person takes an antibiotic, three, or four courses of different drugs. A new mother sensitive bacteria are killed while resistant germs may contract a drug-resistant urinary tract infection are left to grow and multiply—a classic case of that keeps her in the hospital an extra day or more. natural selection. Too-frequent and improper uses of antibiotics are the main causes of today’s increase in How Bacteria Become Drug Resistant drug-resistant bacteria. Bacteria are able to resist drugs through one of several mechanisms. Some develop the ability to Another source of antibiotic resistance originates inactivate or destroy the antibiotic before it can do with the way we raise livestock, fish, and orchard harm. Others can rapidly pump the antibiotic out crops. Almost 70 percent of all the antibiotics of bacterial cells. Still others can change the place produced in the United States are added to animal in the cell that antibiotics target so that the drugs feeds—not to fend off disease but to boost growth. are ineffective. The more these resistant organisms These non-therapeutic uses of antibiotics are a spread, the more they add to the pool of resistance perfect way to cultivate resistant organisms, including genes in all bacteria, raising the odds that these Campylobacter and Salmonella, bacteria that can genes will jump to more and more disease- sicken people who eat meat and poultry products. causing microbes. The story of staph bacteria and The Toll of Resistance Antibiotic resistance has been called one of the antibiotics illustrates the perils world’s most pressing public health problems. of drug resistance. Scottish Almost every type of bacteria has become less bacteriologist Alexander responsive to the antibiotic treatment designed Fleming discovered the first to combat it. And antibiotic resistance affects antibiotic, penicillin, in 1927, everyone’s health in a way that no single disease an achievement for which does. It is a particularly serious problem for patients he was co-awarded a Nobel whose immune systems are compromised, such as Prize in 1945. By the early 1940s, people with HIV/AIDS and patients in critical care the drug was used in patients. But MRSA bacteria. 24

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penicillin-resistant staph bacteria emerged as early • If your health care provider prescribes an antibiotic as 1942. Today, virtually all Staphylococcus aureus for you, do not skip doses and do not save any for the are penicillin resistant. next time you get sick. Complete the prescribed course of treatment, even if you are feeling better. Staph bacteria are commonly carried on the skin or • If you are a hospital patient or have a loved one in the nose of healthy people. MRSA—methicillin- in the hospital, make sure that you and the doctors, resistant Staphylococcus aureus—is a type of staph nurses, support staff, and all visitors wash their hands that is resistant to antibiotics called beta-lactams. In or use a hand sanitizer prior to touching the patient. the past the majority of MRSA infections occurred among patients in hospitals or other health care Chronic Illness and Cancer settings. But drug-resistant staph is also showing up in healthy people who have not been staying in a hospital. If common staph bacteria were to become In the United States, 70 percent of all deaths are due to resistant to all readily available antibiotics, the chronic diseases. Until recently their biological causes practice of medicine would change dramatically. were mostly unknown. Today, growing evidence Any surgery or invasive procedure could bring life- suggests that infections are behind many chronic threatening complications. As was the case in the pre- diseases once thought to be caused by genetic, antibiotic era, even the most minor cuts in the skin environmental, or lifestyle factors. could prove fatal. The human papillomavirus (HPV), for instance, Though this discussion focuses on the evolving causes more than 90 percent of cervical cancer resistance of bacteria to antibiotics, the issue of cases. The hepatitis B virus accounts for more than antimicrobial resistance is actually much broader. 60 percent of liver cancer The resistance of viruses such as HIV and influenza cases. The hepatitis C virus to antivirals and of protozoan parasites to antimalarial causes cirrhosis, end-stage drugs is a huge problem around the globe. Microbes liver disease, and liver cancer. have the capacity to develop resistance, whether they Human herpesvirus 8 causes are bacteria, viruses, or protozoa. Kaposi’s sarcoma, a malignant complication of AIDS. How to Protect Yourself Helicobacter pylori, a spiral- To avoid contracting an antibiotic-resistant infection: shaped bacterium, is the agent of peptic ulcers and gastric • Do not demand an antibiotic when a health care cancer. These examples may be provider says it is not needed. just the tip of the iceberg. • Do not take an antibiotic for a viral infection, such as the common cold. Helicobacter pylori. 25

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example. Doctors used to assume that stress and Clues to Infection spicy foods caused ulcers—and recommended For scientists there are tantalizing clues that a bland diets. Today they simply cure the condition by seemingly chronic disease may be infectious. When prescribing a pair of antibiotics. an illness arises mostly in people whose immune systems are weak, it suggests infection (such as in Another advance in prevention is the hepatitis B Kaposi’s sarcoma following organ transplants). vaccine. Liver cancer is one of the most common When a disease gets better with antibiotics (as cancers in the world and the most common cancer does strep-induced rheumatic fever), it’s likely to in some parts of Asia. With the hepatitis B vaccine be infectious. Another sign of possible infection now included in universal childhood immunization is chronic inflammation, which is a common programs, new cases of this cancer are expected denominator in such diseases as multiple sclerosis, to drop. rheumatoid arthritis, lupus, and other autoimmune diseases. It remains to be proven that any of these diseases have infectious origins, though the possibility certainly exists. The traditional standards for establishing a microbial As even this brief survey of disease threats shows, cause of disease were developed in the 19th century trends in infectious disease are never stagnant. for acute infections such as tuberculosis and anthrax. They reflect dynamic forces in the world, some When it comes to tracking down an infectious source of which are the subject of the next section. The of chronic disease, however, traditional standards microbe–human relationship continues to evolve in may prove to be too restrictive. Sometimes the ways that may not always be predictable, especially suspect bacteria or viruses are difficult to detect or as we exert ever-greater stresses on the planet in our grow in the lab. Or there may be long delays between endless quest to survive and prosper. infection and disease, so that by the time symptoms appear, the agents that caused the original infection may be gone. Some psychiatric illnesses, such as schizophrenia, may have been triggered by infections that occurred just before or after birth. Studies are in progress to address this possibility. New Treatment Approaches Proof that certain infections cause chronic diseases raises the promise of treatment with antibiotics or vaccines. The discovery that infection with H. pylori was associated with peptic ulcers is a well-known 26