What constitutes a living organism? There are four criteria: it must be able to replenish the energy it uses, it must be able to respond to external stimuli, it must be able to reproduce, and it must perform these actions with some degree of organization. There are organisms which perform some of these actions but not all of them, so they cannot be classified as living. These organisms are called viruses.
At its most basic, a virus is a small capsule of protein containing either DNA or RNA. There are two main classifications of virus; animal viruses and viruses which attack bacteria, or bacteriophages. Bacteriophages inject their genome into a bacteria cell, which incorporates with the bacteria’s genome. This new sequence codes for the production of viral proteins which self-assemble into new viruses. Lysosomes destroy the cell membrane which kills the cell. This whole process is called the lytic cycle. There is a similar process called the lysogenic cycle, in which the virus proteins are not created or assembled until some external signal.
Animal viruses operate differently than bacteriophages. An animal virus it admitted into the cell inside of a vesicle. The virus injects its genome into the cytosol, and leaves the nucleus intact. The cell creates the proteins and the new viruses assemble and leave the cell without damaging it. An assembled virus is the lowest free energy state, so it does not require any extra energy to come together.
Viruses do not constitute living things. They do have a high degree of organization, and are able to respond, but they are missing some crucial criteria. They aren’t able to reproduce, because they require the assistance of the cell’s transcription and translation mechanisms to create the proteins they need. They can’t refuel, because they aren’t able to make their own DNA/RNA. Viruses are in sort of a limbo between living and non-living organisms. But who knows? Maybe the very definition of life will change someday in the light of new discoveries.