For zoonotic diseases, which by definition involve the jumping of a pathogen from its natural host to humans, and in some instances extension of its host cycle to include humans, conditions can be described for this simple schema as follows. The likelihood or frequency of transmission events change when the natural host or pathogen changes, humans change, or the ecosystem supporting both changes. Thus, fundamentally the processes influencing transmission of zoonotic pathogens can be described as a consequence of one or a combination of three possible kinds of change: expansion of the habitat or geographic range of a host, of a pathogen or both; expansion of human's habitat or geographic range; or change in the habitat or ecosystem occupied by both humans and the natural host. Evolutionary adaptation of pathogens is omitted from the figure. However, it can be assumed any factor contributing to increased likelihood of transmission, as well as increased population size of hosts and pathogens will contribute to the potential that new, increasingly pathogenic, infectious, antimicrobial resistant variants will emerge. Anthropogenic climate change, while not incorporated in the diagram, can be considered to potentially contribute to disease emergence through its contribution to habitat alteration.