Epidemiology is the study of the distribution and determinants of health-related states or events. It has traditionally been associated with the study of external agents of disease (microbial infectious agents). Events such as the Black Death, which killed about one-third of the population of Europe during the Middle Ages, have left their trace in the collective unconscious and are still largely perceived as being caused by external environmental agents. Progress in medical science, however, has changed the perceptions of epidemiology as new patterns of illness and disease are confronted.
The first transition in epidemiology occurred over a period of about one hundred years during the nineteenth and twentieth centuries, when humankind progressively curbed the major epidemics of infectious diseases. Some, such as smallpox, were completely eradicated, while some, such as measles, diphtheria, and tetanus, have been significantly reduced. Even taking into account the emergence, or reemergence, of diseases like AIDS (acquired immunodeficiency syndrome), Ebola fever, and malaria, infectious diseases, in many parts of the world, are not the immediate universal threat today that they once were.
The reduction of epidemics and infectious diseases has led to a considerable increase in life expectancy. As a result, there has been an increased appearance of manifestations linked to aging (the so-called degenerative, or chronic, diseases, e.g., some cancers, cerebrovascular disease, and certain mental disorders such as Alzheimer's disease). Major changes in behavior and lifestyles have also appeared, albeit more recently, including driving cars, changes in smoking and diet, and an increased pressure related to work or unemployment. These changes have led to new patterns of disease, distinguished by the growth of the "human-made" diseases such as lung cancer, coronary artery disease, and motor-vehicle injury. Many of these diseases, of course, are not new and have complex etiologies, but the shift from the prevalence of infectious diseases to the prevalence of chronic diseases defines this second transition that has occurred in every part of the developed world. In the developing world, this transition is rapidly emerging, putting these countries under a new burden while they are still suffering from the ongoing—and in some cases increasing—onslaught of the acute communicable diseases.
The next transition may be brought about by changes in how individuals relate to disease. Patterns of usage of health services, ranging from the limitation of services on economic grounds to the provision of technologically advanced services such as organ transplantation and in vitro fertilization, are continually shifting. At the same time, efforts made by individuals to manage the consequences of nonfatal impairments and disabilities are also likely to bring changes in the distribution and determinants of health-related states or events. Epidemiologic transitions are, in fact, an ongoing phenomena, with one phase overlapping another. These transitions will continue as the face of disease continues to change.
MICHEL C. THURIAUX
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