Abstract
Most viruses are host-specific and thus, they are closely associated to an animal species. This host specificity is conditioned by the cell susceptibility to the virus, given by the host cell receptors, and by the cell permissivity that relies on the availability of cellular factors required for viral replication. However, for many viruses, this host specificity is not absolute and inter-species transmissions are possible. Viruses which can be transmitted from an animal reservoir to humans are responsible for zoonotic diseases. Human activities and their impact on the natural environment dramatically contribute to emergence of zoonoses. Selection of viral mutations that confer better adaptation of animal viruses to humans may lead to epidemic spread of these diseases.