An emerging pathogen has been defined as the causative agent of an infectious disease whose incidence is increasing following its appearance in a new host population or whose incidence is increasing in an existing population as a result of long-term changes in its underlying epidemiology (Woolhouse and Dye 2001). Although we appear to be in a period where novel diseases are appearing and old diseases are spreading at an unprecedented rate, disease emergence per se is not a new phenomenon. It is almost certain that disease emergence is a routine event in the evolutionary ecology of pathogens, and part of a ubiquitous response of pathogen populations to shifting arrays of host species. While our knowledge of emerging diseases is, for the most part, limited to the time span of the human lineage, this history provides us with a modern reflection of these deeper evolutionary processes, and it is clear from this record that at many times throughout human history, demographic and behavioural changes in society have provided opportunities for pathogens to emerge.