Viral zoonoses cause overt disease in humans and other animals or sile
nt 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 natu
ral host animals and cause no signs of disease, but this balance may b
e upset by many factors, including stress, changing environmental cond
itions and mutations in both viruses and hosts. Infection of an unnatu
ral host may cause clinical disease but no further spread of the virus
. Examples of viral zoonoses both old and new are used to illustrate s
ome of these points, and two will be described in detail - Rift Valley
fever and influenza.