Send us feedback. Copyright 1999 American Society for Microbiology. All rights reserved. When it's a virus.
Viruses are strange things that straddle the fence between living and non-living.
On the one hand, if they're floating around in the air or sitting on a doorknob, they're inert. They're about as alive as a rock. But if they come into contact with a suitable plant, animal or bacterial cell, they spring into action. They infect and take over the cell like pirates hijacking a ship. Some viruses have an additional layer around this coat called an envelope.