ments ripe for the emergence of infectious diseases. Land development for housing or use in agriculture; the creation of dams and reservoirs necessary to maintain water for agricultural use and public consumption; and outdoor recreational activities all bring humans into contact with arthropod vectors, rodents, and other animals capable of transmitting infections. Furthermore, changing climate and weather, as well as natural disasters such as floods and earthquakes, can impact on ecosystems to generate ideal conditions for the transmission of pathogens. The convergence of any number of such factors can create an environment that allows infectious diseases to emerge and become rooted in society.

Whereas the angry sea dissipates to an eventual calm, leaving few witnesses to a meteorological perfect storm, the factors creating a microbial perfect storm can perpetuate and even accelerate its effects—leaving multitudes of people to bear witness and fall victim to its destructive forces. In just two decades, for example, the world has witnessed widespread devastation due to the human immunodeficiency virus (HIV), a pathogen unrecognized before 1981. By 2001, more than 40 million people were estimated to be living with HIV, and 20 million had already died from acquired immunodeficiency syndrome (AIDS), the result of HIV infection.

In 1999, West Nile virus was isolated for the first time in the Western Hemisphere. The infection began with an epicenter in New York; by 2002, nearly 4,000 cases of West Nile encephalitis had been reported in 39 states and the District of Columbia, of which 254 were fatal. Although the first case of West Nile encephalitis was identified in Uganda in 1937, the virus was not considered to be a significant human pathogen because most infections were either mild or asymptomatic. Between 1996 and 1999, however, three major epidemics in urban areas (southern Romania, the Volga region of southern Russia, and the northeastern United States) resulted in hundreds of cases of severe neurological disease and fatal infection from West Nile virus, suggesting a change in the pathogen’s virulence.

In 2001, 22 people in the United States contracted anthrax as innocent victims of an act of bioterrorism. Of these 22, 11 suffered from inhalational anthrax, the most lethal form of disease caused by Bacillus anthracis; 5 deaths resulted. This intentional use of a microbe to cause harm was the sobering realization of a once hypothetical factor in the emergence of a microbial threat. While the anthrax event may be more analogous to artifactual seeding of the clouds, HIV and West Nile virus have emerged as a result of microbial perfect storms—and now continue as turbulent microbial threats to health.

An infectious disease epidemic can be a major destabilizing force for any nation, and endemic infectious diseases sap strength from the population and impede national development. The economic and social instability that typically accompanies or follows in the wake of an infectious disease



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