Izabela Mazur
Izabela Mazur voted up Ray Dart's answer

Michael Oldstone described a virus as "A bit of nucleic acid surrounded by bad news."

For a better (and more scientific) explanation:

A virus is a biological agent that can exist only inside a living cell. It replicates when the cell replicates, or replicates inside the cell. It is not generally considered to be a living thing, … Read more