Characteristics of life: In order for life to survive an organism must be composed of one or more cells, have complex organization patterns, use energy, stabilize internal conditions using homeostasis, be able to grow, and reproduce.
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The only exception is viruses which are not composed of cells but are said to be "living".

Viruses are a microscopic infectious agent that can reproduce only inside a host cell. They are found in almost every ecosystem on earth and are the most abundant type of biological entity on the planet. All viruses have genes made from either DNA or RNA. All have a protein coat that protects these genes. They are an important means of horizontal gene transfer which increases genetic diversity. Plant viruses are often transmitted from plant to plant by insects that feed on sap. Animal viruses cab be carried by blood-sucking insects. For example influenza viruses are spread by coughing and sneezing.

Prions are an infectious agent that is composed of protein. The protein does not itself self-replicate and the process is dependent on the presence of the polypeptide in the host organism. All known prion diseases affect the structure of the brain or other neural tissue.

Viroids are plant pathogens that consist of RNA without the protain coat that is typical for viruses. The human pathogen hepatitis D is similar to viroids.