Given the rarity of spontaneous development of new virus species, the most frequent cause of emergent viruses in humans is zoonosis. This phenomenon is estimated to account for 73% of all emerging or re-emerging pathogens, with viruses playing a disproportionately large role. RNA viruses are particularly frequent, accounting for 37% of emerging and re-emerging pathogens. A broad range of animals - including wild birds, rodents and bats - are associated with zoonotic viruses. It is not possible to predict specific zoonotic events that may be associated with a particular animal reservoir at any given time.