Sometimes written zoonose. Plural is zoonoses. Built from the Greek words zoo, meaning 'animal', and nosos, meaning 'disease'.

The simplest definition of zoonosis is that it is 'a disease that can be transmitted from animals to humans'. A slightly more technical definition is that it is a disease that normally exists in animals, but can infect humans. On other occasions it may be used to mean a disease that can complete its life cycle without a human host, but can infect humans if the opportunity arises. None of these are wrong, although the first is overly simplistic.

Zoonotic diseases are important for a number of reasons:

  • Most of human prehistory has been spent as small bands of hunter-gatherers; these bands were rarely larger then 50 individuals, and were not in contact with other bands very often. Because of this, epidemic diseases, which depend on a constant influx of humans that haven't developed an immune response, tended to burn out after their first run through a population. To survive, a biological pathogen had to be either a chronic infection, staying alive in the host for long periods of time, 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.

Diseases like malaria, schistosomiasis, river blindness, and elephantiasis are not zoonotic, even though they are borne by insects or other animal vectors, because they depend on the human host for part of their life-cycle.