Humans are part of the animal kingdom. This also means that viruses that circulate amongst animals may be transmitted to humans and cause disease in humans. This type of infectious diseases are called zoonotic diseases. Influenza, for example, has come to us from birds, SARS was brought to us by bats, HIV/AIDS is a disease that comes from monkeys.
Once the virus becomes established in humans, they adapt and they manage to circulate among the human population. By looking for viruses in animals we can identify new viruses that may be of concern to human health and acquire new knowledge about these viruses. This knowledge in turn helps us to develop new diagnostics and intervention strategies.