Population and Disease

The study of the relationship between population and disease generally falls under the discipline of epidemiology, although it may also be studied under the relatively new field of disease ecology.

Epidemiology is defined by the World Health Organization (WHO) as “the study of the distribution and determinants of health-related states (including disease), and the application of this study to the control of diseases and other health problems.” This means is that epidemiologists look at where specific health problems are found and how they spread among the population as a whole and among certain subgroups of the population (i.e., distribution) and the factors and causes (i.e., determinants) of these diseases and disorders. They then use this information to help prevent and control diseases and disorders and improve public health. Epidemiologists differ from clinical physicians because they work to understand the steps needed to prevent or cure disease in whole populations or subpopulations, while clinical physicians generally are focused on preventing or curing disease in individual patients.

The word epidemiology comes from three Greek words. Epi means “among”, demos refers to “people”, and ology means “study”. Thus, epidemiology means the study of what (health issues) are among the people.

From the earliest times, physicians have sought to understand the causes of disease, but a systematic study of the causes and spread of disease in entire populations did not take off until the mid-1800s. A massive cholera outbreak occurred in London in late August 1854. Within three days 127 people died. The death rate of those who became ill was about 12%. Before the outbreak was over, more than 600 people died. John Snow (1813–1858), a physician, essentially established the science of epidemiology when he had the idea of mapping the locations where cholera cases occurred. Snow's map showed that most cholera cases clustered around certain city water pumps, while very few cases of cholera occurred in buildings with private wells.

Snow convinced the municipal authorities to disable a public pump in the Soho area of London where the number of cholera cases was highest. Almost immediately, the cholera outbreak subsided in that area. From this, Snow as able to deduce that cholera was caused by contamination of the water. Snow's discovery was the beginning of the science that became epidemiology. His findings were a major event in public health and helped shift the focus of water treatment from taste and clarity to controlling waterborne pathogens.

The history of epidemiology is closely connected to the history of public health. Until the 1940s, epidemiologists were primarily concerned with learning how communicable diseases were cause and how they spread. It was well known that when people were crowded together, disease was common and spread quickly and easily. In studying diseases such as tuberculosis, epidemiologists came to understand that population density alone was not responsible for disease distribution. Socioeconomic factors were also at work.

People who lived in crowded conditions generally were less well nourished, had limited access to health care, were less well educated about disease, often shared living space with disease-carrying animals (rats, insects), and tended to have more physically stressful lives than people who lived in less crowded conditions. Understanding the connection between socioeconomic factors and disease informed public health policy throughout the first half of the twentieth century.

The publication of Rachel Carson's book Silent Spring in 1962 increased awareness of chemicals and toxins in the environment and started the environmental movement. Epidemiology expanded into studying the relationship between environmental toxins other noncommunicable diseases. Examples include the relationship between asthma and air pollution, asbestos exposure and mesothelioma, and ultraviolet light and malignant melanoma. Legal changes in allowable exposure to toxins, banning of certain chemicals, and better product labeling have occurred and will continue to occur because of this research.

Other branches of epidemiology still focus on tracking emergent communicable diseases such as avian influenza and SARS, while still others concentrate on the relationship between social habits, living environments, and disease in various subpopulations. As more and more is learned about the connection between changes in genes and the development of specific diseases, epidemiologists will likely expand their study of the role environmental toxins play as triggers for genetic changes in specific subpopulations.

See also Air pollution ; Asbestos ; Asthma ; Cholera ; Communicable diseases ; Epidemiology ; Melanoma ; Severe Acute Respiratory Syndrome (SARS) ; Smoking ; Tuberculosis .