Ten reasons to exclude viruses from the tree of life. 2009 Nature Reviews Microbiology 7: 306-311
When viruses were discovered, they were accepted as missing links between the inert world and living organisms. However, this idea was soon abandoned as information about their molecular parasitic nature accumulated. Recently, the notion that viruses are living organisms that have had a role in the evolution of some essential features of cells has experienced a renaissance owing to the discovery of unusually large and complex viruses that possess typical cellular genes. Here, we contend that there is strong evidence against the notion that viruses are alive and represent ancient lineages of the tree of life.
Viruses do not reproduce by division, but are replicated by the self-assembly of preformed components. This, not size, differentiates them from cellular living organisms such as bacteria. A virus-infected cell is more like a car factory than a womb.
Also, unlike living organisms, no virus has the means of generating its own energy – they are all energy pirates.