Viruses are neither classified as plants or animals.
Virus is a tiny, basic infectious agent that can only reproduce in live cells of animals, plants, or microorganisms.
Beijerinck originally hypothesized that the virus under investigation was a novel type of infectious agent, dubbed contagium vivum fluidum since it was a living, reproducing creature that was distinct from other species.
Both of these researchers discovered that a disease of tobacco plants could be transmitted by an agent, subsequently dubbed tobacco mosaic virus, that passed through a tiny filter that prevented bacteria from going through.
This virus, as well as others that were later discovered, did not develop on artificial media and was not visible under a light microscope.