Hosts The particular host range of a virus is determined by the virus's requirements for its specific attachment to the host cell and the availability within the potential host of cellular factors required for viral multiplication. Three main classes animal viruses,bacterial viruses (bacteriophages),and plant viruses. Hosts The particular host range of a virus is determined by the virus's requirements for its specific attachment to the host cell and the availability within the potential host of cellular factors required for viral multiplication. Three main classes - animal viruses, bacterial viruses (bacteriophages), and plant viruses