A. Definition and Background.

The Encyclopedia Britannica™ defines virus as:

“Virus, infectious agent of small size and simple composition that can multiply only in living cells of animals, plants, or bacteria.”

Dictionary.com™ defines a virus as:

“An ultramicroscopic (20 to 300 nm in diameter), metabolically inert, infectious agent that replicates only within the cells of living hosts, mainly bacteria, plants, and animals: composed of an RNA or DNA core, a protein coat, and, in more complex types, a surrounding envelope.”

A few viruses are composed of molecules of DNA, but most are a half fragment of a DNA molecule, known as RNA.  It has a lifespan.  It reproduces within a host cell.  And it dies.  Many scientists believe that RNA came first, long before DNA.  

Many viruses attack and kill bacteria.  The phage virus is the most prevalent life form on earth with a mass greater than 76 million blue whales.  The protein that surrounds the outside of a virus is also the same protein that enables the placenta in mammals to stick together.  It is possible that the virus was responsible for allowing life to progress from reptilian eggs to mammals. [Zimmer, Mammals Made by Viruses (2/14/2012) Discover Magazine] 

Viruses are delicate and fragmented.  Those based on RNA do not have the reproduction safeguards that DNA provides.  [Sanjuan et al (2016) Mechanisms of Viral Mutation, Cel Mol Life Sci  73(23) 4433-4448]  [Duffy, Siobain (2018) Why are RNMA virus mutation rates so damn high?, PLOS Biology 16(e)]. This is why most viruses mutate.