This was the first clue to the nature of viruses, genetic entities that lie somewhere in the grey area between living and non-living states.
Viruses depend on the host cells that they infect to reproduce.
When found outside of host cells, viruses exist as a protein coat or capsid , sometimes enclosed within a membrane. The capsid encloses either DNA or RNA which codes for the virus elements. On the right is a micrograph of the virus that causes tobacco mosaic disease in tobacco plants. When it comes into contact with a host cell, a virus can insert its genetic material into its host, literally taking over the host's functions. An infected cell produces more viral protein and genetic material instead of its usual products.