each viral particle consists of a stretch of genetic instructions (written either in DNA or RNA) packaged inside a protein capsule, known as a capsid. The capsid, in some cases, is surrounded by a membranous envelope (like the caramel on a caramel apple), which protects it and helps it catch hold of a cell. A virus can copy itself only by entering a cell, or at least injecting its genome, and commandeering the 3-D printing machinery that turns genetic information into proteins.
