A virus is an infectious submicroscopic creature that only reproduces inside live cells.
Like cell-based life forms, viruses are subject to natural selection. Consequently, some of them may change quickly.
Additionally, because viruses can reproduce considerably more quickly than other life forms, there are more opportunities for them to evolve than there are for other types of life.
All living things, including plants, animals, and microbes like bacteria and archaea, are susceptible to virus infection.