Emerging infectious diseases are recognized as increasing threats to public and animal health, global economy, and social and political stability. Climate change, environmental changes, changes in human demographics and behaviors, and the rise of global trade and travel are most-often-cited drivers for the emergence of infectious diseases in human and animal populations (Schrag and Wiener 1995, Daszak et al. 2001, Dobson and Foufo-poulos 2001, May et al. 2001, Taylor et al. 2001, Antia et al. 2003, IOM 2003, Kuiken et al. 2003, Weiss and McMichael 2004, Wolfe et al. 2005, Woolhouse and Gowtage-Sequeria 2005, Chomel et al. 2007, Woolhouse and Gaunt 2007, Jones et al. 2008). Emerging pathogens are more likely to be zoonotic or vector-borne with a broad host range (Daszak et al. 2000, Taylor et al. 2001, Woolhouse and Gowtage-Sequeria 2005, Jones et al. 2008). Emerging pathogens are also more frequently RNA viruses (Woolhouse and Gowtage-Sequeria 2005), which may better adapt to and establish in novel host species (Antia et al. 2003, andre and Day 2005, Woolhouse et al. 2005). Determining the factors driving disease emergence eventually aims at assisting the prediction of the future emergence of infectious diseases. However, because of the multifactorial nature of the drivers involved (IOM 2003), our ability to predict which pathogens may arise in human or animal populations in the future remains limited (WHO/FAO/OIE 2004). In particular, the ability to identify the animal reservoirs where the greatest risks to human health will originate was deemed improbable (WHO/FAO/OIE 2004). Applying the theory of island biogeography (MacArthur and Wilson 1967) to emerging pathogens identifies (i)interactions between recipient host species and species sources of pathogens (including vector species), (ii) interactions within species sources of pathogens, and (iii) interactions within recipient host species, as mechanisms directly driving disease emergence. Most identified drivers for disease emergence correspond to changes in one or more of these three sets of interactions. Importantly, applying the theory of island biogeography to emerging pathogens assists in predicting from which animal species future zoonotic and vector-borne pathogens will most probably emerge. Identifying known and unknown pathogens such species harbor may bring us one step forward toward predicting which pathogen will next emerge in human populations.