The numbers of emerging infections in the human population that are zoonotic in origin appear to be increasing. The zoonotic reservoir appears to largely consist of mammals, as opposed to other animals. As such, we review, through presenting examples, whether there are certain groups of mammals that may form a greater source of zoonotic infection than others. In addition, we review whether we can identify certain parasites or pathogens that are most likely to emerge as zoonotic infections. In discussing these ideas we propose a five step process describing the nature of the species barrier. For successful zoonotic emergence to occur we conclude that whilst human-mammal exposure may form the first step in emergence the infection process relies on a series of complex intra-host interactions which require further investigation via a multi-disciplinary approach.