A zoonosis ( /ˌzoʊ.əˈnoʊsɨs/) or zoonose is any infectious disease that can be transmitted between species (in some instances, by a vector) from animals to humans or from humans to animals (the latter is sometimes called reverse zoonosis or anthroponosis). In a study of 1415 pathogens known to affect humans, 61% were zoonotic. The emergence of a pathogen into a new host species is called disease invasion.

The emerging interdisciplinary field of conservation medicine, which integrates human and veterinary medicine, and environmental sciences, is largely concerned with zoonoses.

A partial list of agents that can carry infectious organisms that may be zoonotic are listed below. Xenozoonosis is zoonosis transmitted by xenotransplantation (transplantation between species).

Zoonoses can be listed according to the infectious agent:

Other zoonoses might be:

Most of human prehistory was spent as small bands of hunter-gatherers; these bands were rarely larger than 150 individuals, and were not in contact with other bands very often. Because of this, epidemic or pandemic diseases, which depend on a constant influx of humans who have not developed an immune response, tended to burn out after their first run through a population. To survive, a biological pathogen had to be a chronic infection, stay alive in the host for long periods, or have a non-human reservoir in which to live while waiting for new hosts to pass by. In fact, for many 'human' diseases, the human is actually an accidental victim and a dead-end host. (This is the case with rabies, anthrax, tularemia, West Nile virus, and many others). Thus, much of human development has been in relation to zoonotic, not epidemic, diseases.