A virus is an intracellular parasite that can reproduce only by taking over a host cell. A virus consists of a nucleic acid genome enclosed in a protein shell called a capsid. In the virus shown here, the genome consists of DNA, but some viruses have RNA. Some viruses are also covered by a membranous envelope that is derived from the membrane of the host cell. There is usually a lock-and-key fit between the proteins of the capsid and receptors on a particular type of host cell. The virus attaches to a host cell, and viral DNA enters the cell. The viral DNA uses nucleotides and enzymes of the host cell to replicate itself. The viral DNA then commandeers other host cell materials and machinery to transcribe its genes into messenger RNA and translate the RNA messages into capsid proteins. Viral DNA and capsid proteins then assemble into new viruses. Mature viruses leave the host cell, often destroying the cell in the process. The viruses can go on to infect other cells, spreading the viral infection.