The recent worldwide upsurge of zoonotic infectious diseases, involving the resurgence of a growing number of diseases previously believed under control or the emergence of newly recognized diseases, has been attributed to a list of global factors characterized in terms ranging from microbial adaptation and land use to changing ecosystems, breakdown of public health, and poverty (1, 57) . The categorization provided by Gubler (10) , elaborated here based on an expanded view of infectious disease ecology, provides the basis for a schema describing the causal relations involving factors previously identified, along with hypothesized mechanisms.