Viruses are microscopic intracellular obligate parasites with an RNA or DNA genome encased in a virus-coded protein sheath.
They are mobile genetic factors with a long history of co-evolution with their hosts.
They are most likely cellular in origin.
Viruses lack the complex metabolic and biosynthetic apparatus seen in eukaryotic and prokaryotic cells, they require specialist host cells to reproduce.
A virion is a virus particle that has reached full maturity.
The virion's main function is to carry its DNA or RNA genome into the host cell, where it can be expressed (transcribed and translated).
The viral DNA and any accompanying basic proteins are contained in a symmetric protein capsid.