Virus is a parasitic organism composed of a protein coat, nycleic acids enclosed by the coat, and sometimes a few enclosed proteins that facilitate infection of host cells. Attention of biologists was distracted for nearly a century by arguments over whether viruses are organisms.
The disagreement stems largerly from the generalization put forth in the latter half of the nineteenth century that cells are the building blocks of all life.
Viruses are simpler than cells, so, the logic goes, viruses cannot by living organisms. This viewpoint seems best dismissed as semantic dog wagging by the tail of dogma.