Abstract
Viral zoonoses cause overt disease in humans and other animals or silent infections in animals and overt disease in unnatural hosts such as humans. Often the virus and its animal host have evolved together and learned to live together. Infection may spread freely between the natural host animals and cause no signs of disease, but this balance may be upset by many factors, including stress, changing environmental conditions and mutations in both viruses and hosts. Infection of an unnatural host may cause clinical disease but no further spread of the virus. Examples of viral zoonoses both old and new are used to illustrate some of these points, and two will be described in detail--Rift Valley fever and influenza.
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