In each case there may be more than one correct answer.
All viruses are obligate intracellular parasites.
All viruses are surrounded by a lipid bilayer called an envelope. All viruses utilize double-stranded DNA genomes at some point in their life cycle. Among the viruses, some have DNA genomes and some have RNA genomes. All viruses must attach to susceptible host cells as a first step in their reproduction. Several different criteria have been used to classify a virus.