DISEASE IS THE pathological process that presents a characteristic group of symptoms and that establishes the condition as an abnormal entity different from either the normal or other pathological body states. Disease is related to the concept of health, which is broadly defined as a state of dynamic balance, in which an individual or group's capacity to cope with the circumstances of living is at an optimal level. The freedom from disease or the risk of contracting one is only part of the modern concept of health, which involves complete physical, mental, and social well-being. The causes of human disease are varied and result from a combination of environmental, genetic, lifestyle, and socioeconomic factors acting over the lifetime of an individual.
This range of factors and interactions that shape human health have led to a questioning of the classic mechanical model of disease that has dominated the scientific approach guiding research on human health. The mechanical model of disease, as the name implies, only views the organism as a simple machine, instead of a complex system interacting with its environment. In this complex process for preserving health or combating disease, a person or population's environment can shape the behaviors associated with disease.
A changing environment can be a factor in the initiation or spread of disease. Recent research demonstrates the importance of socioeconomic factors in producing disease, and the recognition that these socioeconomic factors, in combination with physical environments (light, temperature, seasonality, and pollution) and biological phenomena, such as pathogens, need to be understood in a holistic way.
The concept of a complex process is key to understanding disease for people inhabiting changing environments, marginal environments, and extreme environments, such as Alaska. Environmental health focuses on the interrelation between human disease and the health of other species in an ecosystem. Contaminated water and air, endocrine disrupters, and even biological terrorism pose important stressors in which human systems have become agents of change that affect rural and natural landscapes, including the movement of organisms and materials. Environmental well-being refers to the ability of the environment to support all life, including human economic and social systems. The efficiency and sustainability of environmental services, the cycling of materials, and the maintenance of organismal balance are needed to reduce the incidence of disease.
Diseases affect homeostasis, which is the feedback that maintains a living organism's body function within limits essential for the body to continue functioning properly, despite external and internal stresses that would move the system away from balance. Diseases that exist in a population for a long time, where they are maintained at a low level, are called endemic. An epidemic occurs when there is a marked increase in the incidence of disease within a region. When an epidemic spreads around the world it becomes a pandemic. For example, the 1918 pandemic is believed to have been related to 30 million deaths. Climate change is expected to increase the spread of disease, especially biologically-transmitted pathogens.
Chronic diseases such as Alzheimer's, emphysema, heart disease, acquired immune deficiency syndrome (AIDS), and cancer are diseases that have a slow onset and last for extended periods of time. Diseases can also be classified by type, such as infectious, developmental, nutritional, radiation, and chemical. Many diseases, such as neoplastic diseases like cancer, involve both genetic components and environmental factors leading to increases in susceptibility. Thermal, chemical, and radiation damage mainly results from a damaging excess of kinetic energy or ionization of molecules in tissues leading to cell death. Free radicals react with molecules in the cell and change their structure, affecting their ability to function. This is the basis for Paul ing's molecular theory of disease. Chemical exposure can stimulate or depress metabolic functions, such as homeostasis, by interfering with enzymes or receptors. Some chemicals produce effects that are characterized as a systemic disease. This disease process affects all of the body's organ systems. Most environmental pollutants, such as mercury, lead, cadmium, arsenic, and persistent organic pollutants are systemic toxins. Systemic pollutants, because they are ingested, inhaled, or absorbed, will be more widely distributed during climate change by floods, fires, and winds.
Most public health problems involve diseases caused by pathogens; but environmental molecules, either natural (metals, metal-containing compounds, or plant secondary products) or anthropogenic (pesticides, pharmaceuticals, or industrial waste) can lead to adverse effects on public health. However, some responses are considered to be adaptive and not disease states, such as the induction of a detoxification enzyme.
Human health is also affected by nutrition, contact with allergens and pathogens, and exposure to contaminants in air, water, and foods. Population density, age distribution, mobility, and other demographic variables, as well as the genetic composition, and cultural and behavioral attributes of individuals, influence the incidence of disease. Thus, understanding the relationships between human health and the environment is critical, particularly given the potential for alteration of ecosystems by socio-economic activity, rapid global change, and climate variability. The World Health Organization monitors disease on a global scale.
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