infectious disease emergence, forest fragmentation, pathogen introduction, pollution, poverty, and human migration. Natural geological determinants of disease primarily relate to the larval stage of vectorborne diseases, when soils and surface water availability factor into insect breeding site availability and quality. These are important but complex issues that are only understood for a few diseases. For example, recent research has shown that forest fragmentation, urban sprawl, and biodiversity loss are linked to increased Lyme disease risk in the northeastern United States (Ostfeld and Keesing, 2000; Schmidt and Ostfeld, 2001). Expansion and changes in agricultural practices are intimately associated with the emergence of Nipah virus in Malaysia (Chua et al., 1999; Lam and Chua, 2002), Cryptosporidiosis in Europe and North America, and a range of foodborne illnesses globally (Rose et al., 2001).

Rates of deforestation have grown exponentially since the beginning of the twentieth century. Driven by rapidly increasing human populations, large swaths of species-rich tropical and temperate forests, as well as prairies, grasslands, and wetlands, have been converted to species-poor agricultural and ranching areas. In parallel with this habitat destruction, there has been an exponential growth in human-wildlife interaction and conflict. This has resulted in exposure to new pathogens for humans, livestock, and wildlife (Wolfe et al., 2000). Deforestation, and the processes that lead to it, have a number of adverse consequences for ecosystems. Deforestation decreases the overall habitat available for wildlife species. It also modifies the structure of environments, for example, by fragmenting habitats into smaller patches separated by agricultural activities or human populations. Increased “edge effect” (from a patchwork of varied land uses) can further promote interaction among pathogens, vectors, and hosts. This edge effect has been well documented in the case of Lyme disease (Glass et al., 1995). Similarly, increased activity in forest habitats (through behavior or occupation) appears to be a major risk factor for contracting leishmaniasis, a disease caused by protozoa and transmitted by sandflies (Weigle et al., 1993). Evidence is mounting that deforestation and ecosystem changes have serious implications for the distribution of many other microorganisms and the health of human, domestic animal, and wildlife populations.

Landscape epidemiology is based on the concept that geology and climate interact to form a characteristic vegetation cover dictated by the available mineral composition of the soil and substrate together with patterns of temperature and precipitation (Fish, 1996). Vegetation, in turn, provides a microclimate (temperature humidity, shade, etc.) and resources (leaves, fruits, nectar, etc.) which determine the species composition and abundance of vertebrates and vectors, which in turn support the natural transmission of specific pathogens. This generalized model is applicable



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