Viruses are intracellular obligate parasites, which means that they cannot replicate or express their genes without the help of a living cell. A single virus particle (virion) is in and of itself essentially inert. It lacks needed components that cells have to reproduce. When a virus infects a cell, it marshals the cell’s ribosomes, enzymes and much of the cellular machinery to replicate. Unlike what we have seen in cellular replication processes such as mitosis and meiosis, viral replication produces many progeny, that when complete, leave the host cell to infect other cells in the organism. Viruses may contain double-stranded DNA, double-stranded RNA, single-stranded DNA or single-stranded RNA. The type of genetic material found in a particular virus depends on the nature and function of the specific virus. The exact nature of what happens after a host is infected varies depending on the nature of the virus. The process for double-stranded DNA, single-stranded DNA, double-stranded RNA and single-stranded RNA viral replication will differ. For example, double-stranded DNA viruses typically must enter the host cell’s nucleus before they can replicate. Single-stranded RNA viruses however, replicate mainly in the host cell’s cytoplasm. Once a virus infects its host and the viral progeny components are produced by the host’s cellular machinery, the assembly of the viral capsid is a non-enzymatic process. It is usually spontaneous. Viruses typically can only infect a limited number of hosts (also known as host range). The “lock and key” mechanism is the most common explanation for this range. Certain proteins on the virus particle must fit certain receptor sites on the particular host’s cell surface.