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.
The central dogma is the underlying idea of molecular biology holds that genetic information can only go from DNA through RNA to protein or from RNA to protein directly.
Retroviruses, primitive viruses, and even prions don't always adhere to the Central Dogma.
These things aren't really considered to be living things though.