A virus is an infectious submicroscopic creature that only reproduces inside live cells.
All living things, including plants, animals, and microbes like bacteria and archaea, are susceptible to virus infection.
A virus lacks mitochondria and other biological organelles.
They rely entirely on their biological hosts for energy and protein synthesis.
They only replicate within the cells of the infected host.
Many viruses, unlike any other microbe, may reproduce themselves in appropriate cells from a single nucleic acid molecule in their genome, i.e., their nucleic acid alone is infectious.