What lessons have researchers learned from studying emergent and re-emerging diseases? First, known viruses are only a small fraction of the total number that are thought to exist in nature. Many diseases, such as the hantavirus of Four Corners, are not actually new but are infecting people in new locations. After a period during which infectious diseases were thought to be well under control, as we approach the end of the century disease-causing agents are breaking out all over the globe. Infectious diseases kill more than 16.5 million people per year, despite major advances in sanitation, medical care, and public education (Platt 1995). Globally, 3.3 million people die from tuberculosis each year. Another two million die from malaria, predominantly, but not exclusively, in tropical regions. Global changes in climate, settlement, land use, industrialization, and urbanization lead to changes in the incidence and pattern of infectious disease.

    If we add chronic and degenerative diseases, such as cancers and cardiovascular illness, to the infectious disease picture, the global health outlook is less than rosy. Cancers of the liver, the esophagus, and the nasopharynx are major causes of death in China, but are very rare in the United States; no one knows why. The incidence rates of degenerative diseases that take years or decades to develop (e.g., cancer and heart disease) are beginning to show the effects of increases in industrial poisons, air pollution, and behavioral changes such as exercise, diet, or the use of more effective pharmaceutical drugs.

    At the same time, the global human population is growing exponentially at more than 1.5% per year. Although this rate is slowing each year as populations move through the demographic transition (discussed in Unit 2), patterns of popoulation growth have consequences for human health. For example, the health status of a population can vary depending upon the proportion of the population that is elderly, young, or of working age. Age is strongly related to immunological status, nutritional status, accumulation of exposures and health risks, propensity for sex or violence, and simple wear and tear on the body. Behavior changes as people age, as do a host of health risks. Reproductive risks are also changing; being pregnant ten, fifteen, or even twenty times can cause serious health problems for women and substantial maternal mortality. These risks are reduced greatly when women have fewer, more widely spaced pregnancies. In those countries early in the demographic transition, death in childbirth and the consequences for children already alive remain serious problems; where fertility rates are low women now outlive men.

    Beyond the impacts of demographic change on human health are the fundamental effects of human activity on the environment. The need to produce more food has resulted in changes in agricultural technology to produce, preserve, and transport food. These technological changes can also pose risks to human health -- fertilizers can pollute water sources and pesticides can be carcinogenic. In addition, new and often marginal land is cleared in forests, highlands, and swamps around the world for agricultural, commercial, and residential use. These land cover changes remove the vegetative sponge that allows rainfall to soak into the soil and recharge the ground water tables and can result in floods. The impoundment of water for generating electricity or for extending irrigation systems can also cause salinization, desertification, and the loss of biodiversity. Each of these changes that affect land and water resources can produce a variety of changes in the ecology of water-related diseases.

    Changes in population mobility associated with technological change and economic globalization also have fundamentally affected the international diffusion of disease. The rate of rural-to-urban migration in lesser developed countries (LDCs) and the increase in air travel and global tourism contribute to the potential rapid spread of disease. An infectious agent that evolves or is transferred from an animal host in a remote rural area, perhaps as the result of forest clearing that brings humans into contact with the host in new areas, has greater potential than ever to reach urban population centers. The result is that even if a certain animal has always carried a particular virus, the increased chance of contact with humans increases the likelihood of an outbreak. The conditions created by rapid urbanization are conducive to the transmission of existing agents and the emergence of new, or newly virulent, agents. By 2025, an estimated 61% of the earth’s population will live in urban areas (WRI 1996), compared to only about 20% a century ago. The difficulties of housing people in cities, and the crowded conditions that often result, continue to create health problems for urban residents. Rural-to-urban migrants often have no choice but to settle in "squatter" housing with no potable water, sewerage, or electricity. Problems from malnutrition can become severe. Crime, violence, and alcohol and drug abuse are frequent companions of these difficult conditions.

 

Is There a Geography to Human Health and Disease?

 
    Is there a geography to human health and disease? To answer this question, we must first define the concepts of health and disease. A dictionary might define health as the condition in which an organism or one of its parts performs its vital functions properly. Health would thus be a state of sound mind and body. Disease could be considered an impairment of this normal state, something that interrupts or modifies the performance of vital functions. Another definition of health is found in the preamble to the 1946 Charter of the World Health Organization that states, "health is a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity." Likewise, disease may better be conceptualized as "that alteration of living cells or tissues that jeopardizes survival in one’s environment" (May 1961). Rene Dubos (1965) introduced the concept of health as the quality of adaptability into the field of medicine and human ecology.

    Throughout history, geographers and the general public have sought connections between health and place. Summer homes outside urban areas in the United States were selected for their healthful environments -- near lakes, hot springs, or in the mountains. Fresh air was thought to cure the ills of urban life. Traditionally, geographers concentrated on researching the relationship between ill-health and environmental conditions. Hippocrates himself addressed the subject in the Fifth Century B.C. Early medical geographies were written by physicians because, before germs were discovered, many diseases were thought to be caused by emanations from the earth or by plants present in air or water. After the age of exploration, physicians and geographers such as Alexander von Humboldt attempted to explain the occurrence of diseases around the earth through links to the terrain, vegetation, and human races. In the 1800’s, ‘medical topography’ began to relate climatic and topographic influences to the health of individuals and societies. To this end, the cartographic representation of the distribution and diffusion of disease was advanced. Mapping the spread of disease geographically continues to be a powerful tool to aid in the discovery of the causes of disease and to suggest points and processes of interventions (Gilbert 1958; Thomas 1992).

    The successful application of medical cartography is at odds with the failed geographic concept of environmental determinism which sees the environment as determining or influencing human activity. The development of possibilism eased the determinists’ strict control of the environment on human activity. Possibilism is a paradigm or way of organizing human/nature interactions, in which the environment acts to restrict human activities to a range of possibilities. Moving even further away from the influence of the environment on human activity, a behavioral approach gained favor. In the behavioral paradigm, humans were viewed as detached from the environment, or alternatively, as able to ameliorate the environment through technological advances. The recent emergence of cultural ecology represents a more holistic framework of human-environment interactions. Cultural ecology incorporates notions drawn from ecosystem analysis and general systems theory to study cultural practices as related to broader movements in ecological and human systems, with the overall goal of ensuring environmental sustainability.

    Geographers, recognizing a need for cross-disciplinary research, have investigated health and disease considering both physical and human factors that together influence bodily and mental health. Two traditions of inquiry have emerged: (1) disease ecology, which investigates the causes of ill-health, and (2) the geography of medical care and its accessibility (Hunter 1974). Health researchers of all kinds classify types of health problems into several broad categories. These are briefly discussed below with examples illustrating the influence of global environmental change.

 
Nutrition

    Proper nutrition is essential to health and well-being. Dietary problems such as vitamin deficiencies exist throughout much of the world including your own town or neighborhood. In many parts of the world, these problems result from a combination of population growth, food scarcity from poor farming and land management techniques, and inequities in access to resources. Famines often result from human-induced droughts and can lead to debilitating diseases of malnutrition, such as kwashiorkor, a protein deficiency disorder in children prevalent in densely populated parts of the world where the diet consists mainly of starchy vegetables, especially cassava (also called manioc or tapioca). Because such a diet is deficient in certain amino acids that make up proteins vital for growth and for maintaining the immune system, the disease’s manifestations include increased susceptibility to infection, skin changes, edema, severely bloated abdomen, diarrhea, and retarded development. Several crops, when eaten as a staple food by the poor between growing seasons, are associated with specific vitamin deficiencies. For example, reliance on corn can lead to pellagra, a disease marked by inflammation and scaling of the skin, digestive disturbances, and sometimes mental disorders. The power milling of rice to remove the brown bran also removes the nutrient thiamin and has caused beriberi to spread like an epidemic in Asia, following the transportation routes of the mills.

    Technological change, urbanization, and the globalization of economic activity can affect nutritional diseases in diverse ways. For example, global economic trends that require women to work outside the home may result in the reduction or elimination of breast feeding and the substitution of bottle feeding. Because breastfeeding is one of the most important preventative measures for diarrheal disease in infants, this reduction has an impact on the health and nutrition of children. In addition, the introduction and development of a cash economy can result in the elimination of local cultivation of vegetables and livestock as money is spent on canned food and alcohol (Meade 1988, 130-132).

    Global environmental changes such as climate change may also affect nutritional diseases. Shifts in precipitation regimes throughout the world (leading to reduced rainfall in some areas and increased rainfall in others) are likely to affect agricultural productivity and food availability. Likewise, the transformation of agriculturally productive land into urban residential or commercial land uses may reduce our ability to feed the earth’s population.

 
Infectious Disease

    Infectious diseases remain the leading cause of death in the world. They may be spread by contagion, person to person, by ingestion of contaminated food or water, or by injection into humans from the bite of insects that have acquired the infectious agent from the blood of other infected people. Many infectious diseases will continue to increase as strains of the disease agents become resistant to medication (such as antibiotic resistant strains of tuberculosis), as people crowd into tighter urban spaces, and as the need for land results in people moving into new, possibly infectious, areas. The processes of their changing geography is explored more fully in Unit 2.

 
Chronic or Degenerative Disease

    As countries and regions become more developed economically, the incidence of chronic or degenerative diseases such as heart disease, increases significantly. In the United States, cardiovascular and other diseases, arising in part from sedentary lifestyles, have surpassed infectious disease as the number one cause of death. Also as areas experience rapid economic growth and an influx of western culture and products, people emulate behavior from other areas that are detrimental to their health (e.g., smoking). Smoking-related illness, already prevalent in the United States, is likely to continue increasing in Asia as smoking rates increase among the younger population.
 

Psychosocial Disease

    Psychosocial disease, often referred to as mental illness, has social, psychological, and biological roots. Stresses and hypertension due to overcrowding, poor working and/or economic conditions fall within this category, as do mental illnesses, which research has increasingly shown to be linked to biochemical processes. During the process of urbanization, in which migrants from diverse cultures move into cities from rural areas, people may feel that they have lost their traditions and their ways of life. These new forms of stress may promote suicide, domestic abuse, and substance abuse.

 
Genetics

    Some health problems are caused by genetics -- that is, they are inherited as a result of a genetic trait. These may be as mild as allergies or as deadly as Huntington’s Disease. Lactose intolerance, a genetic health problem, may affect nutrition and cause diarrhea. Some genetic traits seem to have evolved as a defense against malaria infection. One examples is the sickle shape of blood cells under oxygen tension and the associated anemia among Africans; another is the severe allergic reaction and destruction of blood cells that can result among Mediterranean people from eating fava beans. Although genetic causation is quite rare, genetic susceptibility to health problems that others are more able to resist is more common. The factor that people are susceptible to must be determined. Sometimes geographic pattern offers the best causal clue, as when a carcinogen and cancers are associated with air pollution, eating smoked fish, or proximity to a contaminated river water source. Genetic susceptibility has been suggested for several important diseases such as multiple sclerosis and Parkinson’s Disease, but they remain disease of unknown etiology because the factor to which people are susceptible remains elusive.

 
Biometeorology/Bioclimatology

    Biometerological and bioclimatological factors that influence human health include altitude, solar radiation, cold and heat waves, and the incidence of drought and flooding. The importance of these factors is illustrated by the simple example of vitamin D (needed for the normal development of bones and teeth) that is synthesized in humans with the aid of exposure to the sun or ultraviolet radiation. On a larger scale, the potential impacts of global and regional climate change on human health have been widely studied. Disease patterns may be significantly affected by global warming trends as vectors previously unknown or rare in particular regions survive and thrive in modified climates. Increased incidence of skin disease (particularly cancer) and eye disease are also anticipated as the level of stratospheric ozone is diminished and more ultraviolet radiation penetrates the earth’s atmosphere. Increased temperatures in some areas as a result of climate change and the human-induced greenhouse effect may cause additional heat-related deaths, particularly among the elderly.

 
Environmental Contamination

    Natural and human-made toxins are present in the environment. We now risk contact with dangerous chemicals in soil, water, air, our workplaces, and our homes. Cancers and respiratory ailments are possible consequences of such pollution. Indoor air quality in offices and the problems associated with computer use (eye strain, carpal tunnel syndrome) are also considered environmental factors, as are the poor health conditions associated with substandard housing.

 
Health Care Delivery and Access

    The availability of health care resources, including access to information and physical facilities, is critical for local, regional, national, and international responses to health problems. Health resources also have a cultural dimension including access to modern, "Western," and traditional healing methods and issues of utilization. Do people know what care is available and where? What are the impediments -- economic, social, or cultural -- that prevent people from using available services?

    It should be clear from these few examples that human health is endangered through many different pathways of exposure. One aspect alone does not produce ill-health, and accordingly, geographic research on human health and global change must explore a variety of pathways. Jones and Moon (1993) state that