As organisms, humans have different levels of nutritional or immunologic status, and different strengths and susceptibilities. When we consider biology, the first thing that usually comes to mind is genetics. Genetics is not so important in actually causing disease, which is rare, but rather in explaining genetic susceptibility (how resistant someone is to an infection or how severe her or his reaction is to a health risk). A person can be immune, that is, unable to contract a disease even when infected by the disease agent, because of the ability of the immune system to recognize the agent and destroy it. This may simply be due to previous exposures or it can be the result of genetic differences in enzymes and biological reactions. In fact, even individuals who are susceptible to developing a disease can have varying levels of resistance to it. That is, a person may contract a milder form of the disease because of good nutrition or because of a genetically given balance of enzymes and free elements in the bloodstream that affects the agent's proliferation. Gender and age are also important because these variables are associated with hormonal and other biochemical differences (e.g., different nutritional needs). For example, the major female reproductive hormone, estrogen, is implicated in protecting the cardiovascular system from cholesterol, promoting calcium metabolism and strong bones, and causing breast cancer. Dietary needs for iron, calcium, and other nutrients vary by gender and age.

    The age structure of an entire population (the proportions of the population in the various age categories) is an important component. Over time, people accumulate different experiences and exposures as they play various age-dependent life roles. Age is often important in how one reacts to an infection or trauma. In most medical studies, data must be standardized for age differences because age is related to many factors that affect the incidence of disease.

 
Environment

    Environment in the explanatory framework refers to a host of factors that are place-based and influence a person’s risk of contracting a disease or becoming ill. These include place, scale, the natural environment, and the built environment. On a global scale the human habitat is the ecumene, the inhabitable world. When people think of habitat what often comes to mind is the natural environment: rainfall, forest and other land cover, plants and animals, soils and land forms, altitude, sun, and climate. It includes the composition of the breathable atmosphere, soils, and water resources. This natural world also includes organisms that can affect human health such as mosquitoes and ticks, viruses, bacteria, protozoa, and helminthic parasites.

    Human habitat is more than the natural world, however. We also live within structures and landscapes that humans have constructed. Water and sewerage systems, transportation and communication infrastructure, and institutions such as hospitals and schools are all part of the human habitat. The social environment also contributes to the local habitat; it may contain danger; it may contain a sweat shop; it may contain good music on the radio or a good book; it may contain friends and relatives, or it may not. This whole habitat affects the population that lives within it. People may feel fear or love, for example, while itching from the detergent in their synthetic fiber shirt, breathing air from an automobile's exhaust, basking in the warm, light of spring, and slapping at the season’s first mosquito.

 
Culture and Technology

    Habitat conditions are largely created or modified by culture -- the beliefs, values, and organization of the population. Culture encompasses a range of values, such as those governing education, religion, attitudes toward the environment and technology, and even beliefs about disease causation. Culture is itself affected by habitat, most noticeably by perceptions of space and distance and by the varying experiences of different habitat conditions. Human behavior is greatly affected by culture and is the manifestation of economic constraints, social norms, political structures, and individual psychology.

    Behavior mediated by culture exposes people (individuals, groups, populations) to some hazards or experiences and protects them from others. Therefore, our health is often related to the roles we play (i.e., as child or adult, man or woman, teacher or student, farmer, or executive) and the types of work or behavior associated with them. For example, weeding a vegetable field can expose an individual to pesticides; long hours of cleaning poultry or typing can encourage the development of carpal tunnel syndrome; and selling products in crowded market places can increase exposure to respiratory infections like tuberculosis. It is clear that who develops sicknesses from these experiences (i.e., mutated sperm, nerve damaged hands, or tuberculosis) depends in large part on who it is that does such things. Empowering women to go to school, to control their own reproduction, or to work in industries or the military, affects their exposure to or protection from various health hazards.

    Culture is also an important factor to consider in addressing public health education and responses to disease. Cultural practices that may make a population more susceptible to certain illnesses need to be understood. Public health efforts need to reflect the cultural norms of the people at which they are aimed; otherwise, such efforts are likely to fail.

    Finally, technologies are closely related to culture and have implications for human health. While damming a river to provide water for drinking and agricultural purposes may serve a public need, for example; it may also expose a population to water-related disease. The choice of technologies and the associated health risks can be highly dependent upon cultural context. For example, in France, nuclear energy is the primary source of electricity while in the US, coal burning is much more prevalent. One can argue that this difference reflects in part the different cultural contexts and the related opinions, beliefs, and values about technology, health, and risk.

 
 

Examples of Disease Ecology and Environment-Culture/Technology-Population Interactions

 
    The three factors in the framework above are interrelated; none is entirely separate from the other. The best way to think of these interactions is with reference to a specific geographical context. At any particular place and time, demographic, environmental, social, cultural, and technological factors intersect in specific ways. In other words, it’s probably not possible to determine which one of these factors is the most important. That can only be understood in terms of a specific place and incidence of disease. Even then, a range of primary, secondary, and tertiary interacting factors can probably be identified.

    Before we conclude this section, let’s consider two examples of disease ecology drawn from different conditions to illustrate these interactions. The first example concerns intestinal parasites. Young adult women in many developing countries have high rates of roundworm infection, but low rates of hookworm infection. In contrast, young adult males have high rates of hookworm infection but lower roundworm infection rates. Current male hookworm rates, however, are much lower than they were in previous generations, and in some places the parasite has disappeared. How can this be explained? Both intestinal worms pass their eggs through human defecation, but the transmission pathways to humans differ. Roundworms are transmitted to humans when dried fecal matter in dirt or dust containing roundworm eggs is ingested. Roundworm is thus considered a disease of the toilet and is often transmitted through handling small children. Even if women drink tea (boiled water) as a cultural buffer against water-borne infections and practice good hygiene, the dust-borne roundworm eggs are ingested by small children, deposited by them, and may contaminate the mother. Hookworm, in contrast, hatches from eggs deposited on the soil and the larvae have to survive various habitat conditions: sun, wetness, soil pH, vegetation, and being eaten by arthropods. The larvae are most likely to survive in shaded areas of fields, where, if stepped on, they penetrate the skin of the foot. The larvae is then transported by the blood to the lung, where it is coughed up into the mouth and swallowed. It then metamorphosizes in stomach acid into an adult that hooks on to the intestine and sucks blood, often causing anemia. Hookworm commonly infects young boys who herd goats or water buffalo in the fields. Since worms live for years, they accumulate and reach higher levels in young adult males. Young men today, however, have a potent new cultural buffer -- they are likely to wear thonged rubber soles (flip-flops) even if they can’t afford shoes. This universally accessible rubber barrier has lowered infection rates around the world.

    In a second example, the death rate of males in the US from cardiovascular disease has begun to decrease in recent years, after decades of increase that raised the degenerative disease to the number one cause of death. This is in part because American culture has begun to promote and pursue more healthy behavior. Millions of people have stopped smoking (although smoking rates are again up among teenagers), lowered their fat consumption, and many have even taken up jogging or other aerobic exercise. Mortality rates are responding to these behavior changes. Because access to information and health care resources tends to be greater among better educated and wealthier segments of the population, there is an increasing divergence of mortality rates by economic class.

 
 

The Demographic Transition

 
    Exponential growth of the earth’s human population is one of the driving forces of global change. At mid-century, the earth had a population of around two billion. It is now almost six billion. Depending on the assumptions made and the policies implemented, in 50 years the earth’s population will grow to around nine or ten billion. Yet it is not just population growth, but the associated consumptive needs and wishes (the impact of culture and technology) that result in changes in land use, depletion of resources, increased energy consumption, shortage of housing, and sometimes social and political strife. In health terms, a decrease in deaths from infectious diseases, especially early childhood deaths, enables population totals to grow.

    One theory that scientists and demographers have used to explain global population trends and regional population differences is the demographic transition theory. The theory is based on the historical experiences of Western, industrialized countries and describes the process of change from high birth and death rates to low birth and death rates related to increases in levels of economic development. As this process occurs, a similar transition occurs, known as the epidemiologic transition. The epidemiologic transition theory describes the changes in patterns of health and disease in a society associated with social, economic, and technological developments that occur during the demographic transition. Therefore, the demographic transition (and the associated epidemiologic transition) provides a useful framework for thinking about health and disease geographically. By examining changes in such factors as the causes of death, the age structure of the population, the number of children born, and the types and trends of mobility that occur in different areas, we can better understand world health patterns and global dynamics.

    As mentioned earlier, the demographic transition is the transition from high birth and death rates to low birth and death rates. High birth and death rates, which the human race has experienced for millennia, are rates of over 40 per 1,000 people. That is, for every thousand people in a place, more than 40 die and more than 40 are born in a year. Low birth and death rates (about 20 per 1,000 people) are characteristic of the later stage of the transition. Because the demographic transition theory was developed based on the historical experiences of industrialized countries in Europe and North America, there is some disagreement about its applicability to contemporary, less-developed countries (LDCs). For example, some European countries took several centuries to complete the transition, while some LDCs are proceeding much more quickly. In addition, many European countries experienced a high level of economic growth as they went through the transition, a process that some LDCs have not always replicated.

    From a theoretical perspective, these examples show that there are differences in how the transition proceeds and what causes changes in each population characteristic in different places. Although each place is different, the overall changes in population structure and dynamics that result can usually be predicted. Figure 3 below illustrates the demographic transition and the four general stages of the process. The following sections describe the four stages of the transition, with a specific focus on factors related to the health of a population within each stage.
 

Source: Kuby, M. 1996. Population growth, energy use, and pollution: Understanding the driving forces of global change. Washington, DC: Association of American Geographers.

    During Stage 1, the mobility of the population is mostly limited to local "circulation," (i.e., movements that return home) such as short-distance trips for marketing, religious practices, or animal herding. Almost everyone dies within a few miles of where he or she was born. The only significant migration is at the time of marriage when, according to local cultural norms, either bride or groom move to the home community of their spouse.

    As mortality from infectious disease is reduced, death rates begin to fall and the population moves into Stage 2 of the transition. Safe drinking water, sanitary removal of human waste, improved nutrition and food supply, better housing, protection against diseases transmitted by biting insects (such as screens for windows), and increased vaccinations help reduce death rates from infectious diseases. Birth rates stay high as couples continue to have six or seven children. As a consequence, the population grows through natural increase -- the difference between the birth and death rates (see Figure 3). As population pressure on the land and the need for jobs increases, the population’s mobility changes. People begin to migrate to marginal agricultural lands or frontiers, to urban areas, and/or to international locations either permanently or as contracted wage labor.

    In Stage 3, rural-to-urban migration continues, education and job opportunities for women increase, and consequently couples have fewer children. As birth rates fall, a smaller proportion of the population is composed of children. The population "ages." Degenerative diseases such as heart disease, stroke, cancer, or kidney failure become the predominant causes of death. Marginal agricultural lands may collapse with people moving to cities, but as the transition progresses and fertility falls, the rate of urbanization slows simply because the majority of the population has already migrated to cities. International migration gradually ceases, but circulation continues to increase in intensity, including the added international dimension of tourism and business travel.

    The relationship between the decrease in birth rates in Stage 3 and literacy, urbanization, industrialization, child survival, and the provision of health care and family planning have been the subject of decades of social science research and recent international conferences (in Cairo, the World Population and Development Conference; in Beijing in 1995, the International Conference on Women). Many contentious issues surround the role of governments in promoting lower fertility rates and faster movement through the transition. In Cairo, the world consensus was that education should be extended to girls even in places where they are now not encouraged to go to school. However, women's control of their own reproductive health through access to contraception even without their husbands' knowledge was much more contested. Any such changes in education, economic activity, or the physiologic burden of pregnancy and childbirth would have profound impact of the health of women and children.

    A population reaches the final stage of the transition when births rates fall into equilibrium with the low death rate. Death rates fall below 20 per 1,000 people overall, and infant mortality rates drop to less than 15 per 1,000 births. Most of the deaths in a population at this stage are older people and the predominant cause of death is degenerative diseases. The population structure has aged, with less than 20% under the age of 15 and more than 15% over 65 years of age. The population is also very urban. Instead of serving as a source of out-migration, the country now becomes the destination of in-migration from other countries in the growth stage of the transition. The population is mobile, and circulation for purposes such as recreation, tourism, and business is intense.

 
Population Mobility and the Demographic Transition

    An important implication of the demographic transition for our purposes is the resulting change in population mobility and its effects on people’s health. During the second stage of the demographic transition, migration has three distinct types of destinations. First, people move to the agricultural frontier as farmers search for new agricultural land. Migration up mountain slopes, deep into rainforests, and to the edges of the plowable grasslands can cause considerable environmental degradation and disrupt biological associations, exposing people to the infections of animals through the biting of various arthropod vectors and setting up concentrations of people within which infections can circulate.

    Second, people migrate from fragmented rural farms to cities in search of work and opportunity. This brings susceptible people into large concentrations under settlement conditions conducive to contagious disease.

    Finally, people migrating in search of jobs occurs increasingly on an international scale, with some moving permanently and others for shorter periods. Given that the more developed countries with older age structures need laborers and the less developed countries with younger age structures need jobs, it is not surprising that the greatest redistribution of global population in centuries is now under way. Each year millions of people migrate between countries. More than 100 million people (2% of the world's population) is living outside their country of birth. In addition, in 1995 there were more than 15 million international refugees. At the same time, the circulation of people from economically developed countries to Third World countries for business and recreation has exceeded 200 million. The technology of air transportation is especially important because it has nearly defeated the age-old social controls of quarantine. People today who become infected with a disease in one location can travel around the world, potentially exposing thousands along the way before they themselves even develop symptoms.

    These changing patterns of human mobility expose susceptible people to new infections from ecological disturbance and environmental degradation. Important consequences are the emergence and rapid global diffusion of antibiotic-resistant strains of disease agents and the emergence of new disease agents from other animals and remote places. There is a growing need for long-range surveillance, laboratory capability, and epidemiological response even as we strive to develop new technologies to balance the global changes resulting from human-environment interactions.
 
 
 

Changing Mobility and Environmental Change

 
    The health consequences that may result from global environmental changes are often considered in terms of the impacts to human settlements. In particular, the catastrophic movements of environmental refugees fleeing land degradation or even sea level rise will contribute to the already present housing difficulties for rural-urban migrants in the rapidly expanding cities. The inevitable results including lack of shelter, overcrowding, poor sanitation, poor diet, and malnutrition are clearly detrimental to health. The approach taken in this paragraph, however common, confuses two separate dimensions: population mobility and urban settlement as habitat. It also entirely neglects the important global process of urbanization itself.

    As explained in Unit 1, the majority of the world’s people will live in cities within a decade, and more than 60% will live in cities within 25 years. Several of these cities will have more than 20 million people each. Given the scale and pace of this development and the fact that most of the growth will occur in poorer countries, the built environment as a habitat for contagious disease takes on a new and frightening dimension. At the same time, the mobility of population continues to grow, adding another factor to the global health picture.

    The contagious diseases that spread in conditions of crowding and poor housing include pneumonia, whooping cough, diphtheria, measles, and tuberculosis (the plague of urbanizing 19th century Europe) (see Table 2). To this list we can add a relatively new disease, HIV/AIDS. Many squatter settlements once perceived by government bureaucrats as hopelessly disrupted and chaotic are now recognized as having viable structures of social organization. The people in such settlements usually respond to opportunities to improve their communities and to receive vaccination and medical care when they are offered. With the support of several agencies of the United Nations, governments have largely changed policy from "bulldozing the festering sores" to empowering communities to improve their built environment. Better construction materials, septic tanks, a safe water supply from a stand pipe, access to electricity, and cooking fuels are often provided. One reason for this growing support for slum improvement is that whatever diseases are spawned by conditions in the settlements will not remain contained within them. Rather than the conditions of urban housing and squatter communities themselves, we must also consider the global implications of the larger process they are embedded within -- global changes in mobility.
 

    Table 3 lists several diseases that are associated with water resources and that pose some of the most serious health risks to human populations. Infectious diseases remain the leading cause of death in the world. They are the primary cause of infant and preschool age deaths. Diarrhea, which has many disease agents, is one of the two most important causes of death for those under five. The second, pulmonary infections in general and especially pneumonia, is also deadly to weakened individuals and the elderly. Diarrhea kills by dehydration. The diffusion in the past decade of a simple and affordable new technology, Oral Rehydration Therapy (ORT), has been successful in increasing survival from prolonged diarrhea in young children. ORT uses an inexpensive and readily available mixture of salt, sugar, and boiled water to stop dehydration. Nevertheless, even with increased child survival, the causes of diarrhea are so widespread, common, and varied that it continues to kill.
 

    There is another set of diseases associated with water. Many dangerous disease agents (viruses, bacteria, protozoa, even helminths) do not infect people by being ingested but by being injected by an arthropod. When an arthropod sucks blood from a person (a "blood meal"), it can pick up disease agents with the blood, and later when it sucks blood from another person, it can incidentally inject the agents into the host. Insects and other arthropods that transmit disease like this are called vectors. These insects are not just "mechanical" vectors, like a dirty finger, but rather are biological vectors because they are essential to the life cycle of the disease agents that grow and reproduce within them. Ticks, lice, fleas, and several species of biting flies are associated with the transmission of various diseases, but by far the most dangerous is the mosquito. Only a few of the tens of thousands of species of mosquito are involved in transmitting human disease, but they account for a lot of it. There are also a few diseases, most notably schistosomiasis, in which the disease agent has to spend part of its life cycle in another animal (termed an intermediate host) before again infecting humans.

    There are concerns that global warming will expand the area in which vectored diseases can occur. It is true that a lessening of winter's severity might allow a species of mosquito, for example, to expand its range into new territory. In most cases, however, the arthropod is already present in that territory (in which the disease of concern does not occur) and the disease can currently be transmitted. The mosquito Anopheles quadrimaculatus, for example, is widespread in the US and is quite capable of transmitting malaria, as it did during the nineteenth century from Minnesota to Arizona. Changes in behavior and habitat eliminated malaria from most of the US before people even knew that a protozoa vectored by a mosquito caused it. These changes included building houses of brick, using glass or screen for windows, and developing new transportation technologies that shifted people away from canal and river water to railroads and planes. Mosquito habitats (e.g., wetlands) were fundamentally altered by human activity such as draining or filling for agricultural use. Under warmer conditions arthropods reproduce faster and hence take more frequent blood meals which could result in transmission to more people.

    Future changes in impoundment and irrigation technologies, social organization, and/or the built environment are likely to be more important to the expansion of malaria than a few degrees of warming of the average global temperature per se. The impoundment of water behind dams to generate electric power or provide perennial irrigation creates open and stagnant bodies of water suitable for several species of Anopheles, the mosquitoes that vector malaria. If sea levels rise as a consequence of global climate change and tens of millions of people are displaced from deltas and river valleys, the mass migration of environmental refugees could introduce infections that any of them have to new populations of people and mosquitoes; enough social disruptions might enable malaria to spread to middle latitudes again as the cultural buffers (mosquito-proof housing or indoor water supply) break down. Mosquitoes that breed in containers like old tires or empty cans (e.g., Aedes aegypti, which transmits dengue fever, yellow fever, and many other arboviruses) or those that breed in sewerage-contaminated ditches and puddles (e.g., Culex spp., which transmits filariasis and some encephalitis viruses) will proliferate in disturbed conditions and refugee settlements.

    The impact of global climate change on precipitation regimes and water availability is a much more important issue than warmer temperatures themselves. Any global changes in climate are likely to cause some areas on earth to become wetter and others to become drier than they are currently. Drier conditions will affect not only agriculture, but sanitation and hygiene. Wetter conditions will affect breeding areas for many arthropods, some of which vector human diseases.

    The following classification of diseases into water-borne (and ingested), unwashed (those preventable by washing and hygiene), and water-based (and vectored) follows the approach of the World Health Organization’s clean water programs in the 1980s and is used as a framework for the remaining sections in this unit. These diseases provide good examples of the possible effects of global environmental change on their distribution.
 

 
Water-unwashed Diseases

    Water-unwashed diseases are those that can be prevented by hygiene and sanitation. People with abundant running hot and cold tap water and laundry facilities are rarely troubled by diseases vectored by fleas or lice. The worm eggs that can contaminate dust, money, fruit, and hands can be eliminated by frequent washing, especially before meals. Even several diseases generally associated with respiratory contagion, such as the common cold, are spread most commonly through contaminated materials and surfaces that contact the face through dirty hands. When wells are remote or dry and water is scarce, it is difficult to maintain bodily hygiene, especially for children. It can be almost impossible to get enough water to do laundry frequently or to wash floors or other dust-contaminated surfaces.

 
Water-based Diseases

    Water-based diseases are, for the most part, vectored diseases in which the arthropod vector spends its larval stage in water. By far the largest class of these are the mosquito-borne diseases: protozoa-caused malaria, helminth-caused filariasis, and the numerous and deadly arthropod-borne viruses (arboviruses) such as yellow fever, dengue fever, lassa fever, Rift Valley fever, and all the place-associated types of encephalitis: Japanese, California, and Eastern equine, among others. Female mosquitoes, which do all of the people-biting, consume blood in association with making and laying eggs. They must do this in close proximity to a water source as mosquito larvae live in water. A wide variety of habitats can be involved; some species, for example, require clear water in shaded containers and others require large bodies of organically charged water in the sun. Since mosquitoes have a limited flight range, foci of infection are concentrated around water sites that are used for breeding. This is also true for the biting black flies whose larvae require flowing, oxygen-rich water in streams. River blindness (onchocerciasis) is caused by the microscopic larvae of the filarial worm (a helminth which lives in mated pairs under the skin), which wanders around near the surface of the body waiting for the fly vector (Simulium damnosum) to stick its proboscis in and suck them up. When these worms make their way to the eyes they cause lesions, scarification, and eventually blindness. Because the flies cannot go far from the streams where they are hatched and where they need to lay their own eggs, the disease has been named by its habitat.

    Schistosomiasis, one of the most rapidly spreading serious diseases in the world, is not formally a vectored disease. The schistosome, a fluke (helminth) must spend a life stage inside a host snail. Since the snail does not inject anyone, it is not really a vector. The schistosome, which infests the veins around bladder or intestine, puts out prodigious amounts of eggs through a person's urine or feces into, usually, water. These hatch into a schistosome stage that seek out certain species of snails in which it multiplies and eventually metamorphosizes again. Breaking from the dead snail, the infectious form of the schistosome (called a cercaria) swims in search of a person and then penetrates directly through the skin into a new human host. The snail is an intermediate host, but the system of transmission can be modeled as though it were a vectored disease.

    Finally, in industrialized countries, and increasingly in developing countries, changes in technology have introduced non-biotic contaminants into rivers, air, and soils. Heavy metals such as mercury, cadmium, and lead, which can persist in sediments and accumulate in the food chain, are the most important of these non-biotic contaminants. Long before the water "dies," the fish become unfit for consumption. In North America, such contaminants pose a health hazard from the Great Lakes to the everglades in Florida. Along with sewerage contamination and fertilizer-induced toxic algae "blooms," industrial pollution has caused not only lakes but estuaries, bays, and extensive offshore areas to be closed to fishing. In Eastern European countries of the former Soviet Union, industrial processes have polluted large areas and killed major rivers. These health hazards are spreading to newly industrializing areas that have seen much success in the global economy, especially southeastern China, where agricultural chemicals and industrial poisons have become greater health hazards in the rice paddy than mosquitoes or snails.

    These water-related diseases are especially susceptible to global changes in climate, land cover, and water supply. Extension of irrigation usually expands the territory of schistosomiasis as well as diseases (e.g., encephalitis and filariasis) vectored by mosquitoes that like slow, nutrient-rich irrigation waters. Building reservoirs behind dams creates ideal breeding conditions for the Anopheles mosquitoes that transmit malaria. Deforestation from either commercial logging or agricultural clearing (as part of the mobility transition) removes the trees and their roots that break the fall of the rain and allow it to seep down and recharge the ground water. Without this living sponge, hard tropical rains run off, eroding soil and gathering into sheet wash that turns even normal rainfall amounts into river floodwaters. The flood backwaters often result in outbreaks of mosquito-borne arboviruses. At the same time, in areas where the rainfall fails to soak in, wells may dry up, local droughts sometimes result, and water-borne and water-unwashed diseases can become epidemic. In urban areas, the concentration of rural-urban migrants in squatter settlements without services creates great public health hazards. People forced to live in these settlements are at great risk of contracting water-borne infections and possibly spreading these infections throughout the city. In developed countries, on the other hand, the level of industrial contamination can cause municipalities to truck in water for consumption. As world population becomes more concentrated and urbanized, agricultural production becomes more intensive, and climatic change affects the abundance and distribution of water, water-related diseases are likely to pose serious threats to human health.