agents, in some instances, have migrated and increased in number. Some microbes that are transmitted by insects or other vectors exhibit these characteristics as well; in addition, they may have become resistant to pesticides, which impedes efforts at control. Finally, the environment has changed and will continue to change. Humanity has altered the world's ecology through deforestation, urbanization, and industrialization, which some believe may lead to global climate change. Moreover, the world periodically experiences civil unrest and war, which can lead to regional breakdowns in sanitation, allowing microbes to flourish. Individually and collectively, these and other factors lead to the emergence and reemergence of microbial pathogens.
Infectious diseases remain the major cause of death worldwide (World Health Organization, 1992) and will not be conquered during our lifetimes. With the application of new scientific knowledge, well-planned intervention strategies, adequate resources, and political will, many of these diseases may be prevented by immunization, contained by the use of drugs or vector-control methods, and, in a very few cases, even eradicated—but the majority are likely to persevere. We can also be confident that new diseases will emerge, although it is impossible to predict their individual emergence in time and place. The committee believes that there are steps that can and must be taken to prepare for these eventualities. Its recommendations address both the recognition of and interventions against emerging infectious diseases.
The key to recognizing new or emerging infectious diseases, and to tracking the prevalence of more established ones, is surveillance. A well-designed, well-implemented surveillance program can detect unusual clusters of disease, document the geographic and demographic spread of an outbreak, and estimate the magnitude of the problem. It can also help to describe the natural history of a disease, identify factors responsible for emergence, facilitate laboratory and epidemiological research, and assess the success of specific intervention efforts.
The importance of surveillance to the detection and control of emerging microbial threats cannot be overemphasized. Poor surveillance leaves policymakers and medical and public health professionals with no basis for developing and implementing policies to control the spread of infectious diseases. The committee does not know whether the impact of human immunodeficiency virus (HIV) disease could have been limited if there had been an effective global infectious disease surveillance system in place in the late 1960s or early 1970s. However, without such a system in place, we would have little chance for early detection of emerging diseases in the future.