Virus – A particle which is capable of independent metabolism and reproduction within a living cell. Outside its host cell, a virus is completely inert.

A mature virus (a viron) ranges in size from 20 to 600 nm in diameter. It consists of a core of nucleic acid (DNA or RNA) surrounded by a protein coat (Capsid). Some bear an outer envelope consisting of proteins and lipids (enveloped viruses).

Inside its host cell the virus initiates the synthesis of viral proteins and undergoes replication. The new virons are released once the host cell disintegrates.

Viruses are parasites of animals, plants and some bacteria (bacteriophages).

Antiviral drugs are effective against certain viral diseases and vaccines provide protection against others.

Common animal viruses include: HIV (AIDS), Influenza (Flu) and Herpes simplex (Herpes).