The multiplication of viruses depends on the virus and the infected cell. Viruses seem to have evolved by several routes and no single pattern of replication has prevailed. The cells can be permissive with productive infection or not. The productive cycle of viruses infecting eukaryotic cells exhibit several common steps. The initiation of infection--attachment, penetration and uncoating--sets up the viral gene for its replication and expression. These steps need specific viral enzymes, because the pathway required for the multiplication of the virus is not known by the cell. The last steps are assembly, maturation of the structural proteins, and finally the egress of viruses from the infected cells.