Jonathan Swift's ditty is true even for those smallest of parasites, the viruses. For example, adeno-associated virus is a par-vovirus (parvo means small), which cannot replicate without its larger companion. Similarly, the hepatitis delta agent (an RNA genome related to plant viroids) depends on hepatitis B (a DNA virus) for encapsidation1, the process in which the viral nucleic acid is packaged in protein.
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