Emerging Viruses: Defining the Rules for Viral Traffic

EMERGING VIRUSES: DEFINING THE RULES FOR VIRAL TRAFFIC STEPHEN S. MORSE* The worldwide epidemic of the acquired immunodeficiency syndrome (AIDS) demonstrates that infectious diseases are not a vestige of our premodern past, but, like disease in general, the price we pay for living in the organic world. AIDS came at a time of increasing complacency about infectious diseases. The striking successes achieved with antibiotics, together with widespread application of vaccines for many previously feared viral diseases, made it appear to many physicians and the public that infectious diseases were retreating and would in time be fully conquered. Although this view was disputed by virologists and many specialists in infectious diseases, it had become a commonplace to suggest that infectious diseases were about to become a thing of the past and that chronic, noninfectious diseases should be our major priorities.1 Rudelyjolted by AIDS back into an awareness of infectious diseases, we now find ourselves in a period of great uncertainty, poised for the AIDS of the future. We cannot help but wonder what other catastrophes are waiting to pounce on us. In this paper, I will consider what we now know about the "AIDS of the future." In particular, I would like to discuss the origins of "new" viruses and whether their emergence can be anticipated and prevented. Using human immunodeficiency virus (HIV) and AIDS as a starting point, I will make the following arguments. AIDS and HIV are novel, but biological antecedents and parallels can be found in nature. The The author is supported by grants RR 03 12 1 and RR 01 180 from the National Institutes of Health, DHHS. *The Rockefeller University, 1230 York Avenue, Box 2, New York, New York 10021-6399. 1 Donald A. Henderson, M.D., Dean, School ofPublic Health,Johns Hopkins University (personal communication, 1989) says he recalls a speech given atJohns Hopkins University in 1969, in whioi 'he surgeon general of the United States Public Health Service expressed the optimism typical of this period, stating that infectious diseases were now marginal in the United States and we should therefore turn our attention toward chronic ailments such as cancer and cardiovascular conditions.© 1991 by The University of Chicago. All rights reserved. 003 1-5982/91/3403-0724$01 .00 Perspectives in Biology and Medicine, 34, 3 ¦ Spring 1991 387 novelty of AIDS therefore probably reflects our imperfect knowledge of the natural world more than it does a diabolical new development in viral evolution. In distinction to this, the conditions favoring rapid dissemination of the virus were comparatively recent social developments of great importance. In essence, they served as highways to expedite "viral traffic," from animal sources to humans and from small or isolated human populations to larger groups. This viral traffic, as I call it, is central to the origin of most epidemics of viral disease. Most "new" or "emerging" viruses are the result of changes in traffic patterns that give viruses new highways. Perhaps most important, human actions often precipitate viral emergence. Apart from such obvious human factors as the role of behavior in HIV transmission, many episodes of emergence involving other viruses have been the result of agricultural or environmental changes brought about by human intervention. In a more general sense, human beings therefore bear greater responsibility for emergence, and may have greater ability to influence it, than has been supposed. The emphasis placed by scientists and the public on the diversity of viruses, of which the stress on the novelty of HIV is one example, may have made us oblivious to the recurrent patterns and common features shared by many emerging viruses. In order to identify these common threads, I will consider several examples of other emerging viruses. The conclusions that emerge from such comparative analyses of viral emergence are as follows. Most "new" viruses are of zoonotic (animal) origin and are not really new but rather are existing viruses that have been given new opportunities or new settings. Viral evolution, while a fascinating phenomenon to scientists, has generally proved less important per se as a mechanism of viral emergence than this transfer of existing or slightly modified viruses to new hosts. The optimistic message is that the possibly unpredictable...