A virus is made up of or consists of a nucleic acid in varying quantity which may either be RNA or DNA.
The nucleic acid is surrounded by a protein shell called a capsid. The word capsid comes from the Latin word capsa which means box.The capsid and the nucleic acid within it are together referred to as nucleoprotein.The capsid is made up of small sub units called capsomeres.
In many viruses, the nucleoprotein makes up the whole virus. More complex viruses have one or more further enclosing structures also made mostly of protein. These structures are referred to as envelopes and each envelop is specific to a particular virus.
