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A virus always has at least two parts: an outer capsid composed of protein units, and an inner core of nucleic acid- either DNA or RNA. The viral genome has at most several hundred genes; a human cell contains thousands of genes. The classification of viruses is based on (1) type of nucleic acid, including whether it is single stranded or double stranded, (2) viral size and shape, and (3) the presence or absence of an outer envelope. It is now believed that viruses are derived from the very cell they infect; the nucleic acid of viruses came from their host cell genomes. Therefore, viruses must have evolved after cells came into existence, and new viruses are evolving now. Viruses, like other organisms, can mutate, and this habit can be quite troublesome because a vaccine that is effective today may not be tomorrow. Viruses are specific to a particular host cell because portions of the capsid bind in the lock-and-key manner with a receptor on the host cell plasma membrane. After viral nucleic acid enters the cell, it takes over the metabolic machinery of the host so that more viruses are produced. The lytic cycle may be divided into five stages: attachment penetration, biosynthesis, maturation, and release. During attachm