A virus is a small infectious agent that replicates only inside the living cells of other organisms.
- A virus is a form of life with very simple requirements. The basic needs of a virus are a nucleic acid to be transmitted from generation to generation (the genome) and a messenger RNA to direct the synthesis of viral proteins. The critical viral proteins that the messenger RNA must encode are those that coat the genome and those that help replicate the genome. One of the great surprises of modern virology has been the discovery of the variety of genetic systems that viruses have evolved to satisfy their needs. Among the animal viruses, at least 6 totally different solutions to the basic requirements of a virus have been found.
- David Baltimore, "Viruses, Polymerases and Cancer: Nobel Lecture" (December 12, 1975)