Viruses can be raised in living plants and animals, however, raising these is often difficult and expensive since it requires large, well equipped facilities.  It is easier both space-wise and ethically to rear viruses in other live cultures such as chicken eggs.

Fertile chicken eggs provide a convenient, space-saving incubator for many kinds of animal viruses.  Different viruses can be injected into an egg at different sites and the egg can be easily observed for viral replication throughout the development of the chicken embryo.