The outer shell of a virus is called the capsid. It consists of several monomeric subunits made of protein. The capsid serves three main purposes :

• It protects the genetic material of the virus.
• It determines if a cell is suitable for infection.
• It starts the actual infection by attaching and "opening" the target cell and injecting the genetic material of the virus into the cell.

Once the virus has infected the cell, it will sooner or later start replicating itself, using the "infrastructure" of the infected cell. During this process, the capsid subunits are synthesized according to the genetic material of the virus, using the protein biosynthesis mechanism of the cell. Some viruses will also take a portion of the host cell's cell membrane with them when they depart, enclosing the proteinaceous capsid with viral proteins projecting through it.

The majority of viruses come in three different structures; helical capsids, icosahedral (isometric) capsids, or enveloped. In helical symmetry, the protein subunits are arranged around the circumference of a circle to form a disk. In icosahedral symmetry, the subunits form a quasi-spherical structure. In enveloped viruses, the protein subunits are exposed to the external environment.

External link

[1] from the All the virology on the www website.