The rate of growth of the world’s population is predicted to slow in the 21st century, but since the beginning of the 20th century the number of people in the world has more than quadrupled—from 1.6 billion to nearly 7 billion—and world population is expected to rise to more than 9 billion by 2050.
The sheer fact of all those people striving to achieve a suitable quality of life means that stress on the resources of our planet is bound to increase. Climate change, with its attendant effects on weather patterns; the displacement of human populations due to political conflict or worsening environmental conditions; and the expansion of settlements into former wilderness areas are all global trends that carry with them significant implications for the nature of humans’ relationship with microbes.
Today’s world is a global village, with growing concentrations of people in huge cities, mass migrations forced by social or economic pressures, and accelerating commerce and travel. An estimated 1.8 million airline passengers cross international borders daily, creating routes by which human infections can radiate around the world within hours. The crates and containers in which goods are shipped worldwide provide safe passage for disease vectors and animal pathogens. Building roads in previously roadless areas brings people into contact with new environments and potentially new pathogens. Cruise ship travel has increased, bringing together—often in confined spaces—thousands of people from diverse geographic regions (including countries with immunization requirements that differ significantly from those of the various sites where ships disembark). And as adventure travelers intrude on new environments and make contact with exotic wildlife, they may encounter microbes that have never before been recognized as human pathogens.
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III Global Challenges The rate of growth of Globalization the world’s population Today’s world is a global village, with is predicted to slow in growing concentrations of people in huge the 21st century, but since cities, mass migrations forced by social or economic the beginning of the 20th century pressures, and accelerating commerce and travel. An estimated 1.8 million airline passengers cross the number of people in the world international borders daily, creating routes by which has more than quadrupled—from human infections can radiate around the world 1.6 billion to nearly 7 billion—and within hours. The crates and containers in which goods are shipped worldwide provide safe passage world population is expected to rise for disease vectors and animal pathogens. Building to more than 9 billion by 2050. roads in previously roadless areas brings people into contact with new environments and potentially new pathogens. Cruise ship travel has increased, The sheer fact of all those people striving to achieve bringing together—often in conﬁned spaces— a suitable quality of life means that stress on the thousands of people from diverse geographic resources of our planet is bound to increase. Climate regions (including countries with immunization change, with its attendant effects on weather requirements that differ signiﬁcantly from those of patterns; the displacement of human populations the various sites where ships disembark). And as due to political conﬂict or worsening environmental adventure travelers intrude on new environments conditions; and the expansion of settlements into and make contact with exotic wildlife, they may former wilderness areas are all global trends that encounter microbes that have never before been carry with them signiﬁcant implications for the recognized as human pathogens. nature of humans’ relationship with microbes. 27
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The movement of people around the globe, depicted here in a map of air traffic among the 500 largest international airports, can lead to the rapid spread of infectious disease. In this rapidly shifting and interconnected world, such as mosquitoes and rats. Poor access to health infectious agents continually ﬁnd new niches. care services worsens the spread of infection. The 2009 “swine ﬂu” pandemic starkly illustrated the impact of globalization and air travel on the Globalization of the food supply has spread disease movement of infectious diseases—with the infection caused by bacteria such as Salmonella and E. coli spreading to 30 countries within 6 weeks and to more O157:H7. The United States, for example, imports than 190 countries and territories within months. about 20 percent of its fresh vegetables, 50 percent of its fresh fruits, and more than 80 percent of its ﬁsh and seafood. As wealthy nations demand such foods The human population is undergoing a mass year-round, the increasing reliance on producers migration from the countryside to “megacities.” abroad means that food may be contaminated Throughout history, big cities have been great during harvesting, storage, processing, incubators of infections—with outbreaks and transport—long before it reaches of respiratory, gastrointestinal, overseas markets. meningeal, and skin infections becoming common in Food is not the only globally crowded urban settings. traded product to set off Substandard housing and waves of infection. In 1999 inadequate sewage and the fungus Cryptococcus water management systems gattii emerged on Vancouver incubate disease vectors Sao Paulo, Brazil—a modern 28 megacity.
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Island, British Columbia, Canada, causing a growing epidemic of human and animal infections and deaths. It has since spread to the Paciﬁc Northwestern United States. The fungus, which causes deadly infections of the lung and brain, had previously been found only in tropical or subtropical climates in such regions as Africa, Australia, Southeast Asia, and the South Paciﬁc. The exact States provides a dramatic example of how acute origins of this outbreak remain a mystery, but some weather events can promote disease transmission. researchers suggest that the fungus may have been The outbreak was traced to a steep increase in the introduced through the importation of contaminated population of deer mice that carry the virus—an trees, shoes, wooden pallets, or shipping crates. increase caused by heavy rains after 6 years of drought, which led to an abundance of food sources Climate Change for the deer mice. West Nile virus emerged in the Eastern United States in 1999, during the hottest and Global climate change is expected to contribute driest summer in a century. Subsequent outbreaks in to the worldwide burden of disease and premature the Midwest in 2003, 2005, and 2006 also coincided deaths. Scientists predict that rising average with heat waves. Scientists believe that hot weather temperatures in some regions will change the may speed up both the breeding cycle of mosquitoes transmission dynamics and geographic range of and replication of the virus in insects’ guts. cholera, malaria, dengue fever, and tick-borne diseases. Increased precipitation may raise the Several recent studies have examined the number and productivity of breeding sites for relationship between the occurrence of infectious vectors such as mosquitoes, ticks, and snails. Rising diseases and short-term climate variation, in atmospheric and surface temperatures are also particular the inﬂuence of the El Niño-Southern increasing the intensity, frequency, and duration Oscillation (ENSO) cycle on the transmission of of heavy precipitation and ﬂooding events, which vector-borne and nonvector-borne infections such may raise the risk of diarrheal diseases and vector- as malaria, dengue fever, and cholera. ENSO is borne infections. the irregular cycling of surface water temperatures between warm and cool phases across parts of the A number of diseases—such as malaria, dengue Paciﬁc Ocean. Global climate change is expected to fever, and viral encephalitis infections—are highly intensify ENSO-related climate variability. sensitive to changes in the environment. The 1993 outbreak of hantavirus pulmonary syndrome (a severe Scientists are currently debating the future effects acute respiratory disease) in the Southwestern United of climate change on vector-borne disease. Some 29
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experts predict that many vector-borne diseases forests and plains, shorelines, and tree lines. Because will expand their range to higher elevations and of the massive expansion of human settlements into latitudes in response to global warming; others claim natural, uninhabited ecosystems, these ecological that human impacts on the local ecology, such as transition zones now dominate much of the deforestation and water use and storage, have a far geography of the world’s tropical developing regions. stronger inﬂuence on the frequency and range of vector-borne infections. When humans move into new environments, microbes that occur in the native wildlife population A secondary effect of climate change also promotes without causing apparent ill effects may adapt infectious disease. Human migration often follows and jump to people. Scientists believe that the extreme weather or weather-associated events, Ebola virus—ﬁrst identiﬁed in a western equatorial including hurricanes, cyclones, ﬁres, drought, and province of Sudan and in a nearby region of Zaïre ﬂoods. The risk of disease outbreaks increases after (now the Democratic Republic of the Congo) in such disasters due to population displacement, 1976—naturally resides in the rainforests on the resulting in unsafe food and water, crowding, and African continent and in the Western Paciﬁc. poor access to health care. Laboratory observation has shown that bats experimentally infected with Ebola do not die, and this has raised speculation that these mammals may Ecosystem Disturbances play a role in maintaining the virus in the tropical forest. Closer to home, Lyme disease surfaced when Whenever animals (including humans), plants, abandoned farmland in the Northeastern United and microbes are introduced into new places, they States reverted to fragmented forest land—a perfect can disrupt ecosystems in ways that increase the home for deer and the deer tick that carries the potential for infectious disease outbreaks. Such bacterium associated with Lyme disease. changes can be difﬁcult to predict and even more daunting to prevent. The term “invasive species” is widely used to describe plants and animals that, when introduced to and established in new environments, spread aggressively. Invasions of disease-causing microbes play out in similar ways. The edges or transition zones between two adjacent ecological systems appear to be “hot spots” for disturbance-induced disease emergence. Examples of such transition zones include the border of human settlements, as well as natural transitions between 30
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The clearing and settlement of tropical rainforests has exposed woodcutters, farmers, and ecotourists to new vector-borne diseases. Deforestation also creates new habitats for pathogens and vectors. In South America, for instance, epidemic malaria has broken out in recently clear-cut areas where mosquitoes now thrive. The construction of large dams can cause profound ecological changes that increase vector-borne diseases. In tropical and subtropical nations the development of large water projects has been associated in some areas with a rise in malaria and schistosomiasis, both parasitic infections. Poverty, Migration, and War Refugee camp in Sudan. Throughout history, poverty and infectious disease diseases of poverty such as intestinal worms, Chagas have been intimately connected. In makeshift and disease, and dengue fever. Pneumonia, diarrhea, overcrowded shantytowns and slum neighborhoods and malaria are among the leading causes of death located on the outskirts of major cities in the in the developing world in children under the age developing world, lack of access to clean water of ﬁve. When there is a lapse in political will to and improper sanitation services spread diarrheal support disease prevention efforts, such as childhood diseases. Worldwide, 884 million people do not have vaccinations, disease can emerge rapidly, as seen in access to an adequate water supply, and about three the spread of polio from northern Nigeria to more times that number lack basic sanitation services. An than 20 other countries. estimated 2 million deaths a year can be attributed to unsafe water supplies; about 90 percent of those In addition, developing nations face public health who die from diarrheal diseases are children in hurdles such as weak health care systems and developing nations. long distances to health care facilities. Limited availability of drugs, or widespread use of poor- People in poor nations often suffer from more than quality or counterfeit medications, has led to drug one infection, because poverty breeds many diseases resistance in the poverty-associated infections of at once, including HIV/AIDS, malaria, tuberculosis, HIV, tuberculosis, and malaria. respiratory and intestinal infections, and neglected 31
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Growing numbers of people are moving within and across national borders after being forced from their homes by war, poverty, or famine. According to some estimates, 1 billion people could be displaced by 2050. Displaced people often bring their livestock, plants, or companion animals with them, increasing the variety of pathogens and vectors that accompany such journeys. Such refugees frequently live in crowded, unsafe conditions that exacerbate the transmission of infectious diseases. Rural to urban migration, for example, has led to increased HIV transmission in Africa. Bioterrorism Bioterrorism is the deliberate release of viruses, An investigator carefully examines one of the letters tainted with anthrax following the 2001 attack in the United States. bacteria, toxins, or other agents to cause illness or death in people, animals, or plants. According to experts, the threat of global bioterrorism is hours to several days after exposure, meaning that increasing. In October 2001, bioterrorism became public health ofﬁcials may not notice the attack a reality when letters containing powdered anthrax until it is too late. Deadly pathogens are highly were sent through the U.S. Postal Service. The attack accessible. With the exception of smallpox, they all caused 22 cases of illness, 5 of which resulted in occur naturally in the wild—in soil, air, water, and death, and widespread fear. animals. And the skills and equipment for making a biological weapon are widely known because they Biological agents are in are the same as those required for cutting-edge work some ways the perfect in medicine, agriculture, and other ﬁelds. weapon of terror. They can be spread through High-priority organisms or toxins that pose the the air, water, or food. greatest risk to national security are known as Terrorists may choose Category A agents, according to the National these agents because Institute of Allergy and Infectious Diseases (NIAID). they can be extremely These deadly pathogens could be readily spread difﬁcult to detect and do in the environment or transmitted from person to not cause illness for several Bacillus anthracis, the agent of anthrax. 32
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person, triggering public panic and requiring special NIAID is developing tools to detect and counter public health precautions. the effects of a bioterrorist attack, including vaccines to immunize the public against diseases Many public health ofﬁcials believe that anthrax is caused by bioterrorism agents, diagnostic tests to the bioweapon of greatest concern—although in help ﬁrst responders and other medical personnel the case of the 2001 anthrax mailings in the United rapidly detect exposure and provide treatment, and States there was less morbidity and mortality than therapies to help patients exposed to bioterrorism many feared would occur. The infection is caused agents regain their health. by Bacillus anthracis, a bacterium that forms spores. Anthrax does not spread from person to person but rather by hard-coated bacterial spores that spring to life under the right conditions. Anthrax can cause skin lesions and gastrointestinal disease. Inhalation Awareness of disease threats fostered by changing anthrax is the rarest form of the infection—and may patterns of human existence and behaviors is the ﬁrst be the most difﬁcult to treat. step toward mitigating their effects. The following section explores what we can do, individually and Another disease of concern is smallpox, a serious, collectively, to address risks posed by this evolving contagious, and sometimes fatal infection. Smallpox relationship between humans and microbes. was ofﬁcially declared eradicated from the globe in 1980, after an 11-year WHO vaccination campaign—the ﬁrst human disease to be eliminated as a naturally spread contagion. Once the disease was gone, routine vaccination of the general public was stopped. Today, the virus remains only in laboratory stockpiles. But in the aftermath of the events of September and October 2001, concern has grown that the smallpox virus might be used as an agent of bioterrorism. Another threat—the botulinum toxin—is the most lethal compound known. The nerve toxin is produced by the bacterium Clostridium botulinum. Researchers estimate that as little as a gram of aerosolized botox could kill more than 1.5 million people. 33