severe acute respiratory syndrome (SARS). The rate of disease emergence has increased significantly over this time, and there seem to be parallel problems in wildlife and even plants (Daszak et al., 2000; Anderson et al., 2004; Jones et al., 2008). In wildlife, a fungal disease has caused a series of extinctions of amphibian species globally, and a transmissible cancer threatens extinction of the Tasmanian devil (McCallum et al., 2007). In plants, diseases of crops and trees have been linked to anthropogenic spread through trade, climate change, and other factors (Anderson et al., 2004). What are the commonalities among these seemingly disparate groups? Are there patterns to emergence that might allow us to predict and prevent the next emerging disease? We should strike a note of caution at this point. In his 1998 address to the International Congress on Emerging Infectious Diseases, Professor Fred A. Murphy reminded us that predicting the next emerging disease’s origin or impact is a significant challenge (Murphy, 1998). The biggest obstacle is probably the sheer size of the unknown pathogen diversity in wildlife, livestock, and other reservoir species with potential to infect humans, should we make the right type of contact. Later in this paper, we will present our approach to dealing with this unknown, but first we consider the commonalities in the process of emergence and how they lead us to a potential solution. We focus here on the emergence of new zoonotic diseases from other animal reservoirs.
There are undoubtedly factors that influence a pathogen’s potential to spill over from wildlife to humans. As a simple case in point, rodent-borne zoonotic pathogens (e.g., hantaviruses) require the presence of rodent reservoirs and, although these creatures exist throughout the world, there are certain areas where rodent abundance is greater or the contact with humans is higher. Although this does not tell us exactly where a rodent-borne pathogen will emerge, it does provide an indication of where there is higher risk. In the same vein, substantial molecular phylogenetic evidence points to a Central-West African origin of HIV-1 from chimpanzees, a species widely hunted for bush meat there. The origins of SARS and some Ebola virus outbreaks have also been linked to the consumption of wildlife. It follows that patterns of human hunting, butchering, and consumption of bush meat will likely predict patterns of the emergence of some zoonotic infections. Finally, SARS coronavirus spread rapidly from China to the New World via infected people traveling on planes (Hufnagel et al., 2004). If we examine trends in global air travel, surely that will give us significant predictive power in analyzing where the next new pathogen in people is likely to spread to. In a very general sense, it becomes clear as we look at the source of each emerging pathogen that almost every emerging disease (perhaps every single one) was driven to emerge by some type of change in human behavior, demography, or anthropogenic environmental change. These emerging diseases are not, after all, “natural” events. If this is true, then it follows that we should be able to predict disease emergence by analyzing trends in demographic, socioeconomic, or environmental changes.