A small infectious agent that is unable to replicate outside a living animal cell. Unlike other intracellular obligatory parasites (for example, chlamydiae and rickettsiae), they contain only one kind of nucleic acid, either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), but not both. They do not replicate by binary fission. Instead, they divert the host cell's metabolism into synthesizing viral building blocks, which then self-assemble into new virus particles that are released into the environment. During the process of this synthesis, viruses utilize cellular metabolic energy, many cellular enzymes, and organelles which they themselves are unable to produce. For this reason they are incapable of sustaining an independent synthesis of their own components. Animal viruses are not susceptible to the action of antibiotics. The extracellular virus particle is called a virion, while the name virus is reserved for various phases of the intracellular development.