Three main criteria used to define a virus

1. Sub-microscopic (20 -50 nanometres) obligate intracellular parasites
2. Their genetic element is either RNA or DNA (not both) that is wrapped into a protein coat (capsid)  and some with lipid envelope enclosing the coat
3. Viruses survive and replicate only in living susceptible cells

“A piece of bad news wrapped up in a protein”