Virion
A virion is a complete virus particle outside a host cell, built to protect a viral genome and deliver it into another cell.
What a virion is
A virion is the complete virus particle that exists outside a host cell. It is the packaged, transmissible form of a virus: a genome enclosed by protective structures and, in some viruses, surrounded by an outer envelope. Inside a cell, the same virus is better understood as a set of replicating genomes, proteins, and assembly processes rather than as a finished particle.
What it contains
Every virion carries genetic material, either RNA or DNA, plus proteins that protect or organize that genome. The simplest virions have a genome inside a capsid. More complex virions may include enzymes, matrix or tegument proteins, membrane envelopes, and surface proteins that help the particle attach to a specific kind of host cell.
Virion versus virus
The word virus can refer to the whole biological system: genome replication, host interaction, disease, evolution, and transmission. Virion is narrower. It names the individual particle stage, especially the extracellular infectious form. That distinction helps avoid confusion when discussing what a virus is doing inside a cell versus how it travels between cells or hosts.
Capsid, nucleocapsid, and envelope
A capsid is the protein shell around the genome. The genome together with that shell or associated proteins is often called a nucleocapsid. An envelope, when present, is a lipid membrane around the nucleocapsid or inner protein layers. Enveloped virions may be easier to disrupt with detergents or drying, while non-enveloped virions often rely on sturdier capsids.
How virions assemble
Virion assembly depends on the virus family. Some capsid proteins self-assemble around the genome, while others first form empty precursor shells that later package genetic material. Enveloped virions usually acquire their membrane as they bud through a host-cell membrane. Maturation steps can then reshape the particle so it becomes infectious.
Attachment and entry
A virion is built for delivery. Surface proteins, capsid features, or tail fibers bind molecules on a host cell, placing the particle at the right entry point. After attachment, the genome must cross a barrier: it may be injected, released after capsid disassembly, or delivered after membrane fusion in enveloped viruses.
Why structure varies
Virions differ widely in size, shape, and composition because viruses solve different transmission and entry problems. A bacteriophage can use a head-and-tail structure to inject genetic material into bacteria. Many animal viruses use envelopes and glycoproteins. Small non-enveloped viruses may favor compact, highly stable capsids that tolerate harsh conditions.
Why it matters
Virions are the stage that vaccines, disinfectants, neutralizing antibodies, and many diagnostic tests often encounter. Understanding their structure explains why some viruses spread easily through surfaces, why others are fragile outside the body, and why blocking attachment, fusion, assembly, or maturation can interrupt infection.