can spread has generated a heightened appreciation of the inherent dangers of microbial pathogens. Acquired immunodeficiency syndrome (AIDS) has become the fourth-leading cause of death worldwide in a mere 20 years since its discovery. Today, more than 40 million people are living with infection from the human immunodeficiency virus (HIV), and 20 million people have died from AIDS. In just 3 years since West Nile virus was discovered in the Western Hemisphere, the virus has spread from its epicenter in New York to 39 states (including California), infecting thousands and killing hundreds.

The emergence and spread of microbial threats are driven by a complex set of factors, the convergence of which can lead to consequences of disease much greater than any single factor might suggest. Genetic and biological factors allow microbes to adapt and change, and can make humans more or less susceptible to infections. Changes in the physical environment can impact on the ecology of vectors and animal reservoirs, the transmissibility of microbes, and the activities of humans that expose them to certain threats. Human behavior, both individual and collective, is perhaps the most complex factor in the emergence of disease. Emergence is especially complicated by social, political, and economic factors—including the development of megacities, the disruption of global ecosystems, the expansion of international travel and commerce, and poverty—which ensure that infectious diseases will continue to plague us. Today we also face the threats of intentionally introduced biological agents. The risks to humankind from the willful spread of highly virulent and contagious microbes are considerable, and we in the United States are preparing to defend ourselves with new vaccines, diagnostics, and therapeutics against the many microbes that might be used in a biological attack. We also are cognizant of the need to rebuild public health infrastructure locally and globally as an indispensable means of reacting to such threats.

Can a focus on naturally occurring microbial threats be maintained in the face of expanded efforts to contain the threat of intentional biological attacks? Some may ask which is the greater risk—the intentional use of a microbial agent to cause sudden, massive, and devastating epidemics of disease, or the continued emergence and spread of natural diseases such as tuberculosis, AIDS, malaria, influenza, and multidrug-resistant bacterial infections. It is a tragic reality that hundreds of people die from naturally occurring infections every hour, whereas until now, intentional biological attacks on a major scale have remained a theoretical risk, rife with political as well as technical uncertainties. HIV/AIDS has taught us the importance of remaining vigilant to the devastation of naturally arising epidemics, which can have profound effects not only on individuals, but also on whole nations and regions. The economic and social disruption that often follows

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