Origin, Emergence, and Re-emergence of Zoonoses

Human pathogens emerge from animals. These are zoonoses. There exists ‘infectivity continuum of pathogens’ from animal to human. Transformation of an animal pathogen into a specialized human pathogen depends upon multiple variables that combine in a dynamic process that is not yet fully understood. The species of animal that harbours the pathogen, the nature and the frequency of human interaction with that animal modulate the risk of zoonotic transmission. In absence of these, phylogenetic proximity may be important, e.g. HIV and chimpanzee and human.