Viruses are composed of a central core of nucleic acid surrounded by a capsid, which is made up of capsomers. Viruses exhibit characteristic symmetry:
- Icosahedral in the case of spherical viruses
- Helical in the case of rod-shaped viruses, and
- Complex in the case of the miscellaneous group
In some viruses, the nucleocapsid is covered by an outer-membrane-like structure called the envelope, which is made up of lipoproteins and conceals this symmetry. Enveloped viruses are sensitive to lipid solvents such as ether and chloroform.
Non-enveloped viruses are referred to as naked viruses. Such viruses are not affected by lipid solvents.