Viruses a. are obligate intracellular parasites. The range of cell types that a virus can infect is referred to as its host range.
Bacteriophages are viruses that infect bacteria.
Bacteriophages are very selective about the hosts they will infect. Only a few species of related bacteria are able to be infected by any one strain of bacteriophage. Animal and plant viruses also have very selective host ranges. Most human viruses only infect humans. A viral particle.