Virion

A virion (plural, viria or virions) is an inert virus particle capable of invading a cell. Upon entering the cell, the virion disassembles and the genetic material from the virus takes control of the cell infrastructure, thus enabling the virus to replicate. The genetic material (core, either DNA or RNA, along with occasionally present virus core protein) inside the virion is usually enclosed in a protection shell, known as the capsid.

While the terms "virus" and "virion" are occasionally confused, recently "virion" is used solely to describe the virus structure outside of cells, while the terms "virus/viral" are broader and also include biological properties such as the infectivity of a virion.

A virion consists of one or more nucleic acid genome molecules (single-stranded or double-stranded RNA or DNA) and coatings (a capsid and possibly a viral envelope). The virion may contain other proteins (for example with enzymatic activities) and/or nucleoproteins.

In the vast majority of viruses, the DNA and RNA components are packed into a protein shell, the capsid. The capsid proteins are often differentiated into major and minor capsid proteins (MCP and mCP). In exceptional cases, there are also viruses without a capsid (i.e., true virions), such as the RNA viruses of the Narnaviridae and the viroids of the Pospiviroidae (with the Citrus Exocortis Viroid and the Citrus Bark Crack Viroid).

If the genome consists of several segments, these are usually packaged together in a capsid (e.g., influenza viruses), and in some viruses, the segments can also be individually packaged in their own capsids (e.g., in Nanoviridae).

Since the genome of viruses is relatively simple, the capsid architecture relies on repetition of simple structures, similar to the faces of a polyhedron. Each face in turn is formed by a repetition of simpler sub-units, with the amount of repetitions called a triangulation number (T). Similar capsid structures can be used by many different types of viruses.

In many viruses, the virions have icosahedral symmetry, which can be ideally isometric or elongated. Many virions also have other shapes:

From observations using microscopy, there are indications of many more distinct shapes.