A virion is the extracellular form of a virus, and contains either an RNA or a DNA genome. The virus genome is introduced into a new host cell by infection. The virus redirects the host metabolism in order to replicate.
Enveloped viruses have one or more lipoprotein layers surrounding the nucleocapsid.
The nucleocapsid is arranged in a symmetric fashion, with a precise number, and arrangement of structural subunits surrounding the virus nucleic acid. Although viruses are metabolically inert, in some viruses, one or more enzymes are present within the virion. Such enzymes play a role in the initial stages of the infection process. Although it requires only a single virion to initiate an infectious cycle, not all virus particles are equally infectious. One of the most accurate ways of measuring virus infectivity is by the plaque assay.