Viruses are infectious agents that are too small to be seen
with a light microscope and that are not cells.
* When they invade susceptible host cells, viruses display
some properties of living organisms and so appear to be on
borderline between living and nonliving.
* Viruses can replicate, or multiply, only inside a living host
cell (obligate intracellular parasites).
* Viruses contain DNA or RNA but never both.