As a first step we distinguish between the two broad categories of human factors, 'demographic and societal' and 'disease intervention and policy' suggested previously by Gubler (10) ( Table 1) . This categorization differentiates between factors associated with specific kinds of environments or ecosystems and those involving biological and policy factors not so associated. However, both can be described as part of a single ecological framework involving interaction of systems of essentially natural versus human design, respectively. Our focus is on the first category from the standpoint of how disease emergence is explained by ecological concepts and principles. This includes some relatively new models and theory not previously used to explain the current trend in increasing emerging infectious diseases. We present a general model of zoonotic disease emergence on this basis. We also discuss recent explanations based on complexity theory for how human behavior and ecosystems interact to contribute to disease emergence. Classical ecology, or natural history, has been the basis and mainstay of infectious disease research since its origins with 'Koch's Postulates' and subsequent development of microbiology and zoonotic disease epidemiology during the 19th and 20th Centuries (14, 15) . Throughout much of its early history, zoonotic disease research involved this descriptive, empiricallybased ecology: identifying the life cycle, transmission, incidental and natural hosts of pathogens, along with demographic, life history, dispersal, and habitat attributes of reservoirs and vectors. A substantial theoretical dimension has developed, beginning with Ross's mathematical analysis of malaria transmission (16) and extending to Anderson and May's (17) recent synthesis Infectious Diseases of Humans.