virions

A virion is the complete, infectious extracellular particle of a virus. It is typically metabolically inert outside a host cell, existing to deliver the viral genome to a suitable cell. The virion consists of a nucleic acid genome (DNA or RNA; single- or double-stranded; linear or circular) enclosed in a protective protein shell called a capsid, and in many cases surrounded by a lipid envelope derived from a host membrane and containing viral glycoproteins.

Capsids can exhibit different geometries, including icosahedral and helical symmetry, and some viruses have complex or multipart structures. Virions without a lipid envelope are termed naked or non-enveloped, while enveloped virions possess a lipid envelope with embedded proteins. Some virions also carry additional layers or proteins, such as tegument in herpesviruses, that assist in early infection after entry.

Infection begins when a virion attaches to specific receptors on a susceptible cell, followed by entry through endocytosis or membrane fusion. The genome is released and directed to replicate using host and viral enzymes. New virions are assembled within the cell and released to infect additional cells; enveloped virions often exit by budding, acquiring their envelope in the process, whereas non-enveloped virions are typically released by cell lysis. Outside the host, virion stability varies with structure and environment; naked virions generally resist drying and detergents better than enveloped ones.

Virions encompass viruses that infect animals, plants, and bacteria (bacteriophages). They are central to transmission, host range, and immunogenicity, with surface proteins frequently serving as targets for vaccines and diagnostics.