The notion of the species barrier is sometimes seen as the ultimate rampart that will protect humans against animal viruses, and this can be true for a number of animal viruses which are not able to intitate infection in human cells. However, the species barrier relies on a number of inate features of the immune system which are bypassed in the case of medicinal products (injectables). In addition, the barrier may only be a quantitative issue (quantity of active virus, e.g. the LD 50 of rabies virus in mice is 10 6 times higher by the oral than by the intracerebral route) and is a question of the route of administration (e.g. the mucous membrane is a rather efficient barrier, however, when a virus is inoculated parenterally, the species barrier is no longer valid). Finally, even in the case of an abortive replication cycle, cell transformation via the expression of early viral genes is still possible, eventually leading to a transformed phenotype of non-permissive cells (e.g., mouse cells are transformed by SV40, which in monkey cells leads only to a lytic cycle, or non-permissive cells transformed by bovine polyomavirus) (Eloit, 1997 (Eloit, , 1999 .