eases with microbial infection (e.g, peptic ulcer with Helicobacter pylori, liver cancer with hepatitis B and C viruses, and Lyme arthritis with Borrelia burgdorferi) has deepened respect for the destructive potential of infectious agents (IOM, 2004).

Infectious diseases continue to cause high morbidity and mortality throughout the world, particularly in developing countries. In 2001, infectious diseases accounted for an estimated 26 percent of deaths worldwide (Kindhauser, 2003). Moreover, there are indications that the tide of human conquest over microbial pathogens is turning. Over the last 30 years, 37 new pathogens have been identified as human disease threats, and an estimated 12 percent of known human pathogens have been recognized as either emerging or reemerging (Merell and Falkow, 2004). Having fallen steadily since the turn of the century, the number of deaths attributable to infection in the United States began to increase in the early 1980s, due in large part to the HIV/AIDS pandemic (Armstrong et al., 1999; Lederberg, 2000).

In the face of these challenges, the metaphor of “war” on infectious diseases—characterized by the systematic search for the microbial “cause” of each disease, followed by the development of antimicrobial therapies—can no longer guide biomedical science or clinical medicine. A new paradigm is needed that incorporates a more realistic and detailed picture of the dynamic interactions among and between host organisms and their diverse populations of microbes, only a fraction of which act as pathogens. To explore the crafting of a new metaphor for host-microbe relationships, and to consider how such a new perspective might inform and prioritize biomedical research, the Forum on Microbial Threats of the Institute of Medicine (IOM) convened the workshop, Ending the War Metaphor: The Changing Agenda for Unraveling the Host-Microbe Relationship on March 16 and 17, 2005.

Workshop participants reviewed current knowledge and approaches to studying the best-known host-microbe system—the bacterial inhabitants of the human gut—as well as key findings from studies of microbial communities associated with other mammals, fish, plants, soil, and insects. Participants and discussants also considered the evolutionary and environmental origins of pathogenesis and reviewed recent findings describing how hosts recognize and respond to pathogens. Additional presentations and discussions addressed the complexity of microbial communities and ecological relationships among pathogens, such as zoonoses, that infect multiple hosts. Finally, participants examined the prospects for manipulating host-microbe relationships to promote health and mitigate disease.

The workshop’s primary goal of replacing the war metaphor for infectious disease intervention represents an expansion of the Forum’s focus on microbial threats to health. The perspective adopted herein is one that recognizes the breadth and diversity of host-microbe relationships beyond those relative few that result in overt disease.

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