very concise and apt article for students interested in virology
This was originally written as an Answer to a Question posted to Scientific American Online; however, as what they published was considerably shorter and simpler than what I wrote, I shall post the [now updated] original here.
The answer to this question is not simple, because, while viruses all share the characteristics of being obligate intracellular parasites which use host cell machinery to make their components which then self-assemble to make particles which contain their genomes, they most definitely do not have a single origin, and indeed their origins may be spread out over a considerable period of geological and evolutionary time.
Viruses infect all types of cellular organisms, from Bacteria through Archaea to Eukarya; from E. coli to mushrooms; from amoebae to human beings – and virus particles may even be the single most abundant and varied organisms on the planet, given their abundance in all the waters of…
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