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were injuries, automobile accidents, and so on. This cohort of men was similar to those in the military—soldiers who were preparing to fight. In the last decade, AIDS has shown a remarkably steep slope of increase and is now the leading cause of death among men of prime age. In this case, expecting the unexpected is not only necessary, but also something that must be prepared for.

Emerging infections are more formally defined as infections that appear suddenly in a population or that rapidly increase in prevalence or in some new geographic range. Many of these conditions will be faced by troops deployed to distant places, which is why the military has a long-standing interest in emerging infections.

During the Korean War, almost 4,000 U.S. and UN troops acquired a viral disease called Korean hemorrhagic fever. The virus was not identified until the mid-1970s, and it is now known as Hantavirus, after a river in Korea near where it was first isolated. Almost 400 troops died, many more were incapacitated, and military demoralization occurred. Thus, emerging infections certainly can affect military operations, in addition to having a civilian impact. Soldiers serve as sentinels, intentionally or unintentionally, for emerging infections.

Viral emergence or, in general terms, infectious disease emergence is a process that involves two steps. The first step is introduction. Where do these apparently mysterious viruses or other agents come from, and how are they introduced into the human population? This has long been the most mysterious step.

Pathogen dissemination is the second step. Many infections are introduced into the human population, but most fail to make the leap to wide dissemination. Infections that are geographically localized cause problems for troops and others who are deployed in those areas, but they do not necessarily lead to a worldwide pandemic. Influenza does on occasion, and AIDS certainly has.

A number of factors can be associated with the emergence of a new infection (IOM, 1992a; Morse, 1997). These factors promote either the introduction or the dissemination step, and sometimes they influence both steps. Introduction factors include ecological changes, which involve the changing relationship between humans and their environment. They typically involve land-use changes or people interposing themselves into formerly sequestered natural environments. The result is increased human contact with a natural host. For example, a rodent species carries an infection that is natural to the wild host, but not to humans. As a result of increased rodent–human contact, an apparently new zoonotic infection can be introduced into the human population and disease may occur.

Other factors leading to the introduction of disease include land-use changes, such as clearing the land for agriculture, which may precipitate the introduction of a new infection.

Human demographics and behavior also play a very important role. The role of human behavior is illustrated by the dissemination of HIV and AIDS. Isolated infections can be introduced from a zoonotic source in an isolated

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