those agents best adapted to human transmission are likely to be those that will emerge. Introduction of a disease-causing agent into a new host population and dissemination of the agent within the new host species can occur almost simultaneously, but they are more commonly separated by considerable periods of time. Changes in the environment and in human behavior, as well as other factors, may increase the chances that dissemination will occur.

For familiar, "old" agents, whose spread has been successfully controlled, reemergence is often the result of lapses in public health measures owing to complacency, changes in human behavior that increase person-to-person transmission of an infectious agent, or changes in the ways humans interact with their environment. The return of dengue fever into areas of South and Central America where previously Ae. aegypti had been eradicated and the resurgence of yellow fever in Nigeria, where more than 400 persons were estimated to have died between April 1 and July 14, 1991 (Centers for Disease Control, unpublished data, 1992), reflect the operation of these mechanisms.

THE CONCEPT OF EMERGENCE

Although specific agents are usually associated with individual diseases, historically it is the diseases that usually have been recognized first. With improved techniques for the identification of microbes, however, this situation is changing. The causative agents for many newly emergent diseases are often discovered virtually simultaneously with (or in some cases before) their associated disease syndromes. For this reason, the term emerging microbial threat as used in this report includes both the agent and the disease.

It is important to understand the difference between infection and disease. Infection implies that an agent, such as a virus, has taken up residence in a host and is multiplying within that host—perhaps with no outward signs of disease. Thus, it is possible to be infected with an agent but not have the disease commonly associated with that agent (although disease may develop at a later time).

In discussions about the emergence of "new" diseases, considerable debate has centered on the relative importance of de novo evolution of agents versus the transfer of existing agents to new host populations (so-called microbial traffic). It is sometimes presumed that the appearance of a novel, disease-causing microorganism results from a change in its genetic properties. This is sometimes the case, but there are many instances in which emergence is due to changes in the environment or in human ecology. In fact, environmental changes probably account for most emerging diseases.

For example, despite the fact that many viruses have naturally high rates of mutation, the significance of new variants as a source of new viral



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