America, as well as other temperate regions of the world. Indeed, two cases of locally acquired malaria were recently discovered in Loudon County, Virginia, 30 miles from Washington, D.C. (CDC, 2002c)

The emergence of a microbial threat is a phenomenon in which something has changed—either our perception of a microbial threat, our recognition of a threat, or the true biological expansion of a microbe. An emerging infectious disease is either a newly recognized, clinically distinct infectious disease, or a known infectious disease whose reported incidence is increasing in a given place or among a specific population. As illustrated in the previous section, HIV, TB, and malaria are certainly emerging infections, even though the latter two diseases have been around for centuries. Figure 2-2 and Table 2-2 provide examples of several emerging infectious diseases identified by scientists in the final decades of the twentieth century. These examples include some diseases that have been known for decades, but have reemerged in new geographic locations and/or in newer, more deadly, drug-resistant forms. These and other examples of emerging infectious diseases, including STDs, nosocomial infections, and vector-borne and zoonotic diseases, are discussed in Chapter 3, along with the major factors in their emergence.

We will inevitably see more emerging infections in the future as the factors that lead to emergence become more prevalent and converge with increased frequency. We can only guess at how many more of the microbes in the environment will eventually be found as human pathogens. Even small, isolated events cannot be readily dismissed because of their potential to expand with time. After all, when the initial handful of cases of what would later be termed AIDS first appeared, few could foresee that their affliction would soon become a global catastrophe, threatening the security of entire nations.

Antimicrobial resistance is a paramount microbial threat of the twenty-first century. With the presence of antimicrobial resistance may come a corresponding increase in mortality and morbidity from untreatable disease, an increased risk of the global spread of drug-resistant pathogens, a rise in the health care costs associated with the need for multidrug therapy and longer and more frequent hospital stays, and the costs of research and development of alternative drugs. For example, efforts to control each of the three major global infectious diseases discussed earlier—AIDS, TB, and malaria—are seriously thwarted by the rise of antimicrobial resistance.