Globalization is defined as the development of an increasingly integrated world economy marked by the free flow of trade, labor, communications, technology, and capital across national boundaries. First used in 1930, the word has come to include the worldwide extent of cultural and intellectual as well as economic trends. In the context of epidemiology, globalization is associated with the increasingly rapid spread of emerging and reemerging diseases and the need for correspondingly rapid responses to these threats to health.
Although some historians of medicine date the globalization of disease only from the nineteenth century, others maintain that the process began as soon as humans domesticated animals that were large enough to enable them to ride rather than to walk on foot, and to carry packages or draw carts loaded with goods that could be sold or traded. There is evidence from chariot burials that humans were using horses for agriculture, transport, and warfare as early as 2000 BCE, and using camels as pack animals from about 1500 BCE. Saddles that allowed humans to ride camels were introduced about 1200 BCE.
In addition to land travel, humans began to travel by water as early as 3000 BCE, as several ancient Egyptian river boats dating from that period have been discovered. As early as 2000 BCE, the Egyptians were taking cats aboard their boats to control rodents, and to help the sailors catch waterfowl. By the fifth century BCE, the Greeks had mastered the art of building warships that could sail across the Mediterranean, and by the second century BCE the Romans had built a navy that helped them destroy Carthage, their North African rival for control of the Mediterranean trade routes. In Asia, the Chinese were using sailing vessels for travel and trade by the second century CE.
What is evident even in the ancient world is the close association between humans and animals, a major factor in the emergence of diseases, and the equally close association of trade and competition leading to warfare.
Globalization has taken place in stages over the course of the last two millennia.
In terms of contacts between different cultures as well as the exchange of goods and information, globalization can be traced back to the first century BCE. The Chinese had developed the art of making silk centuries earlier, but it was not until the reign of Augustus that silk reached the Roman Empire, where it was regarded as an exotic luxury. Roman traders traveled eastward across central Asia in search of Chinese silk, and Chinese traders came westward to purchase Roman glassware, perfumes, and horses. Other goods that were traded during the period from 150 BCE to 1450 CE included spices, jade and other precious stones, leather goods, metalwork, and other types of art. At its height, the Silk Road (composed of various land routes; the name was coined by Ferdinand von Richthofen, 1833–1905, a German geographer) stretched from Korea across Asia to Italy and southern France. In addition to material goods, two major religions traveled along the Silk Road, with Christianity spreading eastward into India and Buddhism moving westward into central Asia.
Unfortunately, diseases accompanied the traders and missionaries on their journeys between the West and the Far East. The first major pandemic of bubonic plague, the Plague of Justinian (541–542, with recurrent outbreaks up through 800), broke out at the same time that Europeans learned to breed silkworms and make silk for themselves. The second major pandemic, the Black Death (1346–1353), is thought to have begun in central Asia in what is now Kyrgyzstan and traveled along the Silk Road westward to India, the Middle East, Turkey, and Europe.
The Age of Discovery or Age of Exploration is a phrase used to describe the period from the second half of the fifteenth century through the eighteenth, when Europeans traveled both westward to the Americas and eastward and southward to Africa, eastern Asia, and Australia. These lands were new to Europeans, although all except Antarctica were inhabited. This second phase of globalization was made possible by such technological advances as sailing ships strong enough to cross the Atlantic and Pacific Oceans; the invention of the printing press, which speeded up the spread of information and led to the first daily newspapers by the early 1700s; the importing of livestock from the Americas to England and the beginning of selective cattle and dog breeding; and the replacement of swords and crossbows with firearms (guns and cannon). These and other inventions gave Europeans many advantages over the native tribes of North and South America, the Aztecs in Mexico, and the aboriginal inhabitants of Australia. France, Spain, the Netherlands, and Great Britain raced to acquire colonial empires and competed with one another for large parts of India, North America, South America, and Oceania.
The wars of conquest as well as the intra-European wars of competition helped to spread disease, as pathogens unknown to the original inhabitants of the colonized lands swept through their populations and decimated them. Smallpox and measles were particularly deadly in the Americas; in some places, as many as 90% of the native population died. In return, the Europeans acquired syphilis from the Americas and tuberculosis, malaria, and dengue from the African slave trade. In addition, the growing international trade in plants and animals led to the emergence of new diseases as some disease organisms crossed the species barrier from animals to humans. There were two important developments in public health during this period, however: the first was the discovery of a vaccination against smallpox; and the other was the emergence of medical journals that allowed doctors of the period to share information about new diseases and treatments.
The medical discoveries and innovations of these two centuries made scientific medicine possible. Hypodermic needles and surgical anesthesia were developed in the 1840s, epidemiology and bacteriology emerged as medical specialties in the 1850s, the germ theory of disease led to the recognition of the need for infection control in the 1860s, and the increased frequency of travel during the period of European colonialism required doctors to study diseases unknown in developed countries and to seek treatments for them. The twentieth century brought the discovery of such vital medications as insulin, antibiotics, pain relievers, antidepressants and antipsychotics, antimalarial and antiparasitic drugs, and antiviral and anticancer compounds, and such new techniques of healing as organ transplantation and gene therapy. On the other hand, these two centuries witnessed the eruption of seven cholera and four influenza pandemics as well as the emergence of such previously unknown diseases as the viral hemorrhagic fevers and HIV infection/AIDS.
The other major development of the nineteenth and twentieth centuries was the formation of national and international organizations to deal with emerging and reemerging diseases. Many of these bodies were started to treat sick and wounded soldiers. The first of these was the British Sanitary Commission, organized to help veterans of the Crimean War (1853–1856). It was followed in 1861 by the United States Sanitary Commission (USSC), which organized medical and nursing care, hospital supplies, mobile kitchens, and other health-related necessities for Union troops during the Civil War. It became obvious after the war that the prevention and control of disease was an ongoing task; the forerunner of the National Institutes of Health was founded in 1887, to be followed by the Public Health Service in 1889, and the Centers for Disease Control and Prevention (CDC), which began as the Office of Malaria Control, in 1946. The CDC's mission was soon expanded to include global health. In addition to partnering with the World Health Organization, the CDC's structure includes a Division of Global HIV and TB, a Division of Parasitic Diseases and Malaria, a Division of Global Health Protection, and a Global Immunization Division.
The first of 14 International Sanitary Conferences was held in Paris in 1851 to establish consistent standards of quarantine for the control of cholera, yellow fever, and bubonic plague. The last of the 14 conferences took place in 1903. In spite of the interruptions of World Wars I and II, the conferences set an important precedent for the establishment of the World Health Organization (WHO) in 1948. The WHO is now recognized as the foremost international institution leading and coordinating the responses of member countries to health crises. Its role in the eradication of smallpox and its technical assistance to developing countries in managing or preventing local epidemics have made it indispensable in dealing with emerging and reemerging diseases.
Globalization affects the demographics of infectious disease in a number of ways. One is the fact that the speed and ease of modern travel, particularly by air, makes it possible for any disease to leapfrog countries and continents in the space of hours. As an example of the globalization of a vector-borne disease, epidemiologists think that West Nile virus entered the United States in 1999 via mosquitoes that had gained entrance to airplane wheel wells while the planes were waiting to take off from airports in the Middle East. Airborne, blood-borne, waterborne, and direct-contact diseases can spread by means of infected bus, train, and airline passengers; SARS spread across Hong Kong through passengers on the city bus lines and from Hong Kong to Canada by air travelers. The Ebola outbreak in the United States in 2014, though small, was another example of the role of air travel in spreading diseases within hours.
The precise demographics of travel-related infections are difficult to determine. Although statistics are usually expressed as the number of events per 100,000 travelers, infectious diseases vary so widely in their incubation periods, severity, and specific symptoms that an infected traveler may well return home before symptoms appear; hence he or she will not be included in the surveillance statistics of the country where the disease was contracted.
Increased population densities and lack of clean water and sanitation in many parts of the world fuel the spread of emergent diseases. Although about 60% of all human pathogens are zoonotic, approximately 75% of emergent infectious diseases are of animal origin. As expanding populations push into previously remote areas, they encounter new pathogens and parasites. Farming practices encroach on wildlife habitat and put humans and animals in closer contact, increasing the opportunities for diseases to jump from animals to humans. Food practices, including the consumption of bushmeat, and religious and cultural practices have also contributed to the emergence of zoonotic diseases. Large-scale environmental changes, including deforestation and habitat loss, have eliminated predators and favored disease vectors such as mosquitoes, ticks, and rodents. A warming planet is enabling pathogens, their animal reservoirs, and their vectors to expand their ranges to higher elevations and latitudes. Climate change is expected to significantly affect emergent diseases in the future.
Microorganisms and insects can evolve resistance to drugs and pesticides very quickly. Bacteria readily transfer antibiotic-resistance genes between species. Viruses recombine and evolve with breathtaking speed. The overuse of antibiotics and insecticides promotes natural selection for resistant organisms. Antibiotics are indiscriminately fed to livestock and poultry and prescribed for viral diseases against which they are completely ineffective. Antimalarial medications such as chloroquine were given to millions of people, regardless of whether they had malarial symptoms. DDT and other insecticides have been in widespread and excessive use. As a result, the rare organisms with natural resistance to drugs and chemicals survive and rapidly spread their resistance genes throughout the population.
The World Health Organization (WHO) monitors emergent diseases around the globe. It maintains a global early-warning system for outbreaks of major animal diseases, especially zoonoses that have the potential to spread to humans.
Many individual states track mosquito- and tickborne diseases, and some monitor insects for emerging viruses such as dengue and WNV. However, public health response to emergent diseases has lagged because of budget cuts and the politicization of climate change. Although many states are developing plans for climate change adaptation, those plans do not prioritize emergent diseases associated with global warming.
There are two major aspects of public health related to the globalization of trade and travel. The first is controlling outbreaks of contagious and zoonotic diseases in the United States, including reemerging diseases as well as those new to the Western Hemisphere. The CDC's National Center for Emerging and Zoonotic Infectious Diseases (NCEZID) is responsible for responding to domestic outbreaks, such as the recent reemergence of dengue in southern Florida or the newly discovered Bourbon virus identified in Kansas in 2014, as well as such international concerns as the appearance of the Zika virus in the Americas, the cholera epidemic in Haiti, and the Ebola epidemic of 2013–2016. NCEZID also oversees the 20 quarantine stations at United States airports, seaports, and land borders that screen incoming travelers and animals for infectious diseases.
The second aspect is the emergence of travel medicine as a distinct medical specialty to care for the health needs of tourists as well as business and diplomatic travelers. Travel medicine covers four main areas: preventive care (vaccination and advice about specific destinations); assistance with obtaining treatment abroad and emergency evacuation if needed; subspecialties that include high-altitude, wilderness, and expedition medicine; and access to medical care provided by travel insurance. The International Society of Travel Medicine and the American Society of Tropical Medicine and Hygiene post lists of clinics on their websites that can help travelers who fall ill after they return to the United States.
Another aspect of the boom in international tourism made possible by globalization that requires mention relates to two specific types of tourism that present their own disease-related issues. The first is medical tourism, defined as traveling to a country other than one's own for medical or surgical treatment. Although many medical tourists in the recent past were wealthy people traveling from less developed countries to highly developed countries for specialized treatment unavailable in their homelands, as of 2017 medical tourists were more likely to be middle-class people from developed countries seeking less expensive health care abroad, most often dental care, plastic surgery, knee or hip replacements, or fertility treatments. The most popular destinations for American medical tourists were Thailand, Mexico, Singapore, India, Malaysia, Cuba, Brazil, Argentina, Panama, and Costa Rica. A major risk to health of medical tourism is the possibility of contracting an infectious disease that is rare in the United States but is endemic or more commonly encountered abroad, particularly malaria, tuberculosis, HIV infection, methicillin-resistant staphylococcal (MRSA) infections, influenza, hepatitis A, and typhoid fever. In addition, the patient's body will be weakened by the stress of travel and surgery or treatment, and thus more susceptible to infection.
The second type of tourism made popular by globalization but associated with a high risk of infectious disease is sex tourism. According to the CDC, between 5% and 50% of adult tourists have casual consensual sexual encounters while traveling, which considerably increase the risk of contracting a sexually transmitted infection. It is significant that the Yellow Book, the CDC's guide to travel health published every two years, now contains a section on the health risks of sex tourism.
Legal issues for U.S. citizens typically include obtaining required vaccinations before traveling abroad. Many countries require proof of vaccination or specific immunizations before allowing entry. A table of recommended or required vaccinations is available at the Merck Manual website. The WHO is the sole issuer of an international certification of vaccination.
Other regulatory issues concern inspection of plants, animals, and goods entering the United States from countries known to have poor sanitation or a history of periodic outbreaks of contagious diseases, especially vector-borne diseases. These inspections are carried out by agencies within the U.S. Department of Agriculture (USDA).
Last, the CDC, the USDA, and the U.S. Department of Health and Human Services (HHS) maintain a list of select agents (bacteria, viruses, and toxins that are considered severe threats to public health and safety). While only two agents on the list, the reconstituted 1918 influenza virus and Yersinia pestis, are causes of past international health crises, laboratory storage and research involving these agents are subject to stringent regulations and controls.
The World Health Organization is still the major international resource responding to the impact of globalization on infectious diseases. In 2005, the WHO established a new version of the International Health Regulations (IHR). The 2005 revision was drawn up in response to evidence from the SARS outbreak of 2002 to 2003 that member states did not contain the novel virus as quickly or effectively as they should have. A third edition of International Health Regulations (2005) was published in 2016. According to the WHO, the purpose of the IHR is “to prevent, protect against, control and provide a public health response to the international spread of disease in ways that are commensurate with and restricted to public health risks, and which avoid unnecessary interference with international traffic and trade.” There are two major problems that have been identified with the WHO's approach, however. One is the organization's top-down bureaucratic structure. As of 2017, the WHO's structure was better prepared to deal with short-term local health emergencies than to manage rapidly developing worldwide health crises. The second issue is the WHO's preference for technocratic solutions to health crises that are not always well suited to the cultures and societies of developing countries.
The WHO has a critical role in international travel, however, because it is the sole issuer of the so-called Carte Jaune or Yellow Card, a bilingual French/English international certificate of vaccination (ICV). After it is filled out by a physician, the ICV should be kept with the traveler's passport.
Globalization and emerging diseases is a complex topic because of its interdisciplinary as well as its worldwide scope. While there are many books on the general subject, most students will find it helpful to choose a specific case study of an emerging or reemerging disease and trace the path of its spread from one country to another, along with the means of its spread. Frontline ( PBS.org ) produced a documentary describing the SARS outbreak of 2002–2003. In addition, the BBC has produced a documentary about the 2013–2014 Ebola epidemic that is particularly revealing about the slowness of the WHO's response to the outbreak and the reasons for it.
Although such agencies as the CDC and the WHO do their best to anticipate emerging and reemerging infectious diseases, it is unlikely that these diseases will ever be completely eliminated. One reason is the rapid pace of genetic mutation in some disease organisms, the influenza virus being the most notable example. The WHO calls an annual conference each February in order to decide which strains of human flu viruses should be targeted by the vaccines prepared for the next seasonal outbreak. Other problems related to genetic mutations in disease organisms include growing resistance to antibiotics among bacteria and the reemergence of fungal diseases among people with HIV infection.
Another reason why infectious diseases are persistent problems is the fact that between 75% and 80% of human infectious diseases are zoonotic in origin; however, worldwide elimination of entire animal species is neither practicable nor desirable. Natural as well as human-made changes in the environment will continue to influence the genetic characteristics, number, and geographic distribution of animals, including the reservoirs and vectors of diseases. It is entirely possible that new diseases will emerge in the future from animal reservoirs as yet unknown.
Last, there are certain characteristics of the human animal itself that guarantee the long-term survival of infectious diseases: curiosity about the unknown, competitiveness in regard to resources, an interest in adventure or even danger, and a tendency to make decisions based on emotion rather than careful thought. As the section on the risks of travel in the CDC's Yellow Book indicates, people vary widely in their perception of risk and their tolerance of it, and there will always be those who choose “to boldly go where no one has gone before.”
See also Antimicrobial resistance ; Ebola virus disease ; Ecological degradation; Epidemiology ; Pandemics ; Severe acute respiratory syndrome (SARS) .
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Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), 1600 Clifton Rd., Atlanta, GA, United States, 30329, (800) CDC-INFO (232-4636), https://wwwn.cdc.gov/dcs/ContactUs/Form , http://www.cdc.gov .
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Rebecca J. Frey, PhD