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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 :

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.