An organism is any living thing. It must grow, develop, reproduce, maintain homeostasis, and demonstrate adaptation. Overall, a living thing is a body made up of organs, organelles, or other parts that work together to carry on the various processes of life.
Opinions differ on whether viruses are a form of life, or organic structures that interact with living organisms. They have been described as "organisms at the edge of life", since they resemble organisms in that they possess genes and evolve by natural selection, and reproduce by creating multiple copies of themselves through self-assembly. They lack genes but have a cellular structure, which is often seen as the basic unit of life. However, they do not have their own metabolism and require a host cell to make new products (although some bacterial species are considered living organisms despite the same limitation). Furthermore, accepted forms of life use cell division to reproduce, whereas viruses spontaneously assemble within cells.
A prion is a disease-causing agent that is neither bacterial nor fungal nor viral and contains no genetic material. It is a protein (not an organism) that occurs normally in a harmless form. By folding into an aberrant shape, the normal prion turns into a rogue agent. It then coopts other normal prions to become rogue prions.
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