Zoonosis is any infectious disease that can be transmitted from animals, both wild and domestic, to humans. The word is derived from Greek words zoon (animal) and nosos (disease). Many serious diseases fall under this category.
Some agents that can carry zoonoses include:
The plural of zoonosis is zoonoses, from which an alternative singular zoonose is derived by back-formation.
The simplest definition of zoonosis is a disease that can be transmitted from animals to humans. A slightly more technical definition 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. 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 or pandemic 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.
Many modern diseases, even epidemic diseases, started out as zoonotic diseases. It is hard to be certain which diseases jumped from other animals to humans, but there is good evidence that measles, smallpox, influenza, and diphtheria came to us this way. AIDS, the common cold, and tuberculosis may also have started in other species.
In modern days, zoonoses are of practical interest because they tend to appear suddenly and be particularly virulent - the West Nile virus appeared in the United States in 1999 in the New York City area, and moved through the country in the summer of 2002, causing much distress. The plague was a zoonosis, as are salmonella, tetanus, Rocky Mountain spotted fever, and Lyme disease.
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.