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I use the acronym of MR. GOSEA to help recall the characteristic of all living things: maintain homeostasis, reproduce, grow, organization (cells --> tissue --> organ --> organ system --> organism), respond to stimuli, use or make energy and adapt. 

Viruses are made of a protein capsid that houses genetic material.  Therefore they are not made of cells, which are considered the smallest unit of life. For this reason, viruses are not considered living. 

Viruses are also not considered to be living because they cannot reproduce on their own. Viruses must invade a preexisting cell and use that cell's genetic to undergo protein synthesis in order to replicate additional body parts that later come together to make more viruses. The cell then becomes so full of new viruses that it explodes. All the viruses that were created within the host cell are then free to invade other cells. This is called the lytic life cycle (lyse = to explode). 

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The question about defining viruses as living organisms or not is a question that has been around for some time and continues to be debated by biologists.  To a very real extent, the answer depends on how you define what life is.  Many people use a set of criteria to define life and most include the criterion that life includes the ability to reproduce.  Viruses cannot reproduce on their own.  They must use the cells of a host organism in order to replicate their viral DNA and produce a new virus.  While many parasitic organisms use a host to facilitate their life processes, a virus has to use the actual cell replication machinery to replicate its DNA.  So unlike a parasite which uses the host as a source of food and energy to replicate, the virus cannot replicate on its own without using the host cell.  So if you use the ability to self-reproduce as a criterion for life, depending on how you interpret it will depend on your view if viruses are alive or not.  It really is a personal judgement call.

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