Viruses (virions) are noncellular parasites. The minimum requirement to be a virus is to have (1) a capsid (the protein coat) and (2) a core structure of either DNA or RNA. Many viruses, however, have additional specialized structures (such as a basal_plate or tail_fibers).

Viruses that attack bacteria are called bacteriophages or phages. They have a sheath that connects the capsid (which contains DNA) with the base plate. The base plate physically supports the tail fibers. Tail fibers help the virus make a physical connection with the surface of the host organism.

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