Epidemiologic Reviews Copyright © 1996 by The Johns Hopkins University School of Hygiene and Public Health All rights reserved Vol. 18, No. 1 Printed in U.S.A. Developing an Integrated Epidemiologic Approach to Emerging Infectious Diseases Stephen S. Morse 1 and James M. Hughes 2 Epidemiology has been a major contributor to the success of the disease control efforts of the past cen- tury, culminating in such signal triumphs as the global eradication of smallpox and the eradication of polio from the Western Hemisphere. However, in recent years, partly because these very successes led to a pervasive optimism about infectious diseases in the future, there has been a waning interest in infectious disease epidemiology even though infectious diseases remain the leading cause of death worldwide and an important cause of death in the United States (1-3). In addition, the tragic recent explosion of the acquired immunodeficiency syndrome (AIDS) pandemic and the emergence of multiple drug resistant tuberculosis forcefully reminded us that infectious diseases could not be relegated to the past. AIDS, like many of the plagues of the past, falls into the category of emerging infections, seemingly new diseases that appear suddenly and unexpectedly. Emerging infections can be defined as those that either have newly appeared in a population or that are rapidly increasing their incidence or expanding their geo- graphic range (4, 5). Other recent examples include hantavirus pulmonary syndrome, Lyme disease, hem- orrhagic colitis, and hemolytic uremic syndrome (re- sulting from a foodborne infection caused by certain strains of Escherichia coli), and Ebola hemorrhagic fever in Africa (6). Past scourges can also recur and are referred to as reemerging diseases, which are often conventionally understood and well recognized public health threats that have increased or reappeared be- cause previously active public health measures have lapsed or sanitary infrastructure has deteriorated. Spe- cific factors precipitating disease emergence can be identified in virtually all cases (2, 4, 5). These include Received and accepted for publication February 20, 1996. Abbreviations: AIDS, acquired immunodeficiency syndrome; HIV, human immunodeficiency virus. 1 Columbia University, School of Public Health, New York, NY. 2 Director, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, GA. Reprint requests to Dr. Stephen S. Morse, Division of Epidemi- ology, Columbia University School of Public Health, 600 West 168th Street (PH-18), New York, NY 10032. ecological or environmental factors that place people in increased contact with a pathogen or its natural host, and environmental, social, demographic, and behav- ioral factors that promote establishment and dissemi- nation of a pathogen previously introduced into a smaller population. The conditions of modern life and changes in human behaviors make these factors more prevalent, giving reason to expect emerging diseases to increase. His- torically, "new" diseases have appeared and spread as by-products of exploration, trade, or warfare, when the movement of people, animals, or goods brought geo- graphically isolated infections to new grounds (7). In the nineteenth century, steamships carried cholera to Europe and Africa. Today, trucks, freighters, and air- planes have largely replaced caravans and steamships, allowing even richer opportunities for infections to emerge and spread efficiently. Speed of travel and global reach of infections are borne out by studies modeling the spread of influenza epidemics (8) and of human immunodeficiency virus (HIV) (9, 10), as well as by the actual progress of known epidemics. Other factors are also allowing emerging infections to appear at increasing rates and could facilitate wider and more rapid spread. In many parts of the world, economic conditions are encouraging the mass move- ment of workers from rural areas to cities. It has been estimated that, largely as a result of this migration, by the year 2025, 65 percent of the world population, including 61 percent of the population in developing regions, will live in cities (11). The phenomenon of rural to urban migration can allow infections arising in isolated rural areas, which may once have remained unrecognized and localized, to reach larger popula- tions, with the city serving an amplifying function. An infection can become further disseminated when other migrants return home with an infection acquired in the city, a pattern now being observed with HIV in Asia. The history of HIV, in fact, is a case in point. HIV may have originated as a zoonotic introduction in a place where people were in contact with the natural host (12). After its probable first move from a rural area into a city, HIV-1 spread along highways to other Downloaded from https://academic.oup.com/epirev/article-abstract/18/1/1/447161 by guest on 11 June 2020