cofactor for the disease to actually take root (Stephens et al., 1998). In other words, regardless of its genetic prowess, the pathogen still must be able to reach its animal (or human) host or vector.

Given today’s rapid pace of economic development and enormous scale of ecological changes, understanding how environmental factors are impacting on the emergence of infectious diseases has assumed an added urgency. To the pressing issues of environmental conservation, natural resource utilization, population growth, and economic development can be added the need to understand the interplay of these processes with the emergence of infectious diseases. Such environmental and ecological factors are playing an increasingly important role in disease emergence. In general, changes in the environment tend to have the greatest influence on the transmission of microbial agents that are waterborne, airborne, foodborne, or vector-borne, or that have an animal reservoir.

Vector Ecology

Pathogens transmitted by mosquitoes and their arthropod allies sicken millions of people each year, cause inestimable morbidity in humans and animals around the globe, and remain major barriers to social and economic development in much of the tropical world. Of the ten diseases targeted by WHO for special control programs, seven have arthropod vectors (WHO 2003a). Many of these diseases—for example, dengue, yellow fever, and malaria—which were controlled to a substantial degree, are now resurgent in many formerly endemic areas. Malaria continues to afflict much of the tropical world and causes an estimated 1.5 million to 2 million deaths per year. More than 2.5 billion people are at risk for dengue virus infection; 100 million cases of dengue are estimated to occur annually, and the incidence of dengue hemorrhagic fever is increasing rapidly throughout the tropics. Yellow fever virus has recently caused major epidemics in Africa and South America (Gubler, 2001; Monath, 2001), and sylvatic reservoirs in these areas provide an ongoing threat for its reintroduction into Aedes aegypti–infested metropolitan areas throughout the world. Ae. aegypti is also the principal vector of the dengue viruses. Vector-borne diseases continue to emerge in new areas and/or to resurge throughout the world, even in areas where they were previously controlled. Many newly emerged pathogens and diseases, including Sin Nombre and Andes viruses (and a plethora of other newly discovered hantaviruses), Guanarito, Lyme disease, and ehrlichiosis all have rodent hosts and/or arthropod vectors (Mills et al., 1999; Gubler, 1998; Gratz, 1999). Others, such as the Seoul, dengue, Japanese encephalitis, West Nile, and Rift Valley fever (RVF) viruses (see Box 3-5), have demonstrated their ability to emerge in new or

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