The concepts of nidality and one-medicine provide the opportunity to explore the natural history of agents of disease; the inter-relatedness of human activity, animal industry, and free-ranging animal populations. Application of the principles espoused by the Medicine and Public Health initiative provides a framework for human medical clinicians, public health practitioners, and the veterinary medicine community to interact and address the threat of bioterrorism in a reasoned and scientific manner based on effective risk assessment. Disease ecology and the global emergence of zoonotic pathogens The incidence and frequency of epidemic transmission of zoonotic diseases, both known and newly recognized, has increased dramatically in the past 30 years. It is thought that this dramatic disease emergence is primarily the result of the social, demographic, and environmental transformation that has occurred globally since World War II. However, the causal linkages have not been elucidated. Investigating emerging zoonotic pathogens as an ecological phenomenon can provide significant insights as to why some of these pathogens have jumped species and caused major epidemics in humans. A review of concepts and theory from biological ecology and of causal factors in disease emergence previously described suggests a general model of global zoonotic disease emergence. The model links demographic and societal factors to land use and land cover change whose associated ecological factors help explain disease emergence. The scale and magnitude of these changes are more significant than those associated with climate change, the effects of which are largely not yet understood. Unfortunately, the complex character and non-linear behavior of the human-natural systems in which host-pathogen systems are embedded makes specific incidences of disease emergence or epidemics inherently difficult to predict. Employing a complex systems analytical approach, however, may show how a few key ecological variables and system properties, including the adaptive capacity of institutions, explains the emergence of infectious diseases and how an integrated, multi-level approach to zoonotic disease control can reduce risk. The growing problem of globally emerging infectious diseases (EIDs) has prompted a substantial effort by the biomedical research establishment to identify the causes and recommend action. As reported in the most recent of a series of volumes (1), a main finding is that the current episode of global infectious disease emergence is the result of a convergence of factors involving complex interactions among numerous variables. This includes genetic, biological, social, economic, political, ecological, and physical environmental factors, and calls for an interdisciplinary research agenda. It is also concluded that "human development and large scale social phenomena are closely associated to infectious disease threats at a global level," which points to the need for research focused on "social and ecological factors affecting infectious disease emergence" (1) .