A viral envelope is a lipid membrane surrounding the internal structures of some virus particles. It contains host-derived lipids together with viral membrane proteins, often glycoproteins. It is structurally distinct from a capsid, the protein-based packaging associated with the viral genome.[ref]
Orthomyxoviridae report by the International Committee on Taxonomy of Viruses. Virion and Fig. 1, lipid membrane and internal components
Formation of the envelope connects viral assembly to the organization of a host biological cell. In a 1978 experiment with polarized epithelial cells, influenza particles emerged from the free apical surface, whereas vesicular stomatitis virus acquired its envelope at the basolateral membrane. The same cellular setting therefore supported different sites of viral budding for different viruses.[ref]
This observation makes the envelope a useful probe of membrane organization. Its host-derived material does not mean it is an arbitrary piece of unchanged host membrane: the completed particle has an organized set of viral components. Identifying where budding occurs and identifying which components enter the particle are related but different experimental questions.
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