Originally Posted by Xurtio

The origin of viruses is an interesting subject.

Nature magazine published a review geared towards the public for it:

Origin of Viruses | Learn Science at Scitable

Whether viruses are living or not is still a matter of debate. The most important question, in my opinion, is to ask whether you can really define something like life in such a binary manner in the first place. Maybe it's more appropriate to talk about lifeness and say that viruses have a small amount of it, but not quite to the level of cellular-based life. In other words, maybe we should model life as a spectrum instead of a yes/no question.

Originally Posted by Frank Ryan

WHEN, in 2001, the human genome was sequenced for the first time, we were confronted by several surprises. One was the sheer lack of genes: where we had anticipated perhaps 100,000 there were actually as few as 20,000. A bigger surprise came from analysis of the genetic sequences, which revealed that these genes made up a mere 1.5 per cent of the genome. This is dwarfed by DNA deriving from viruses, which amounts to roughly 9 per cent.

On top of that, huge chunks of the genome are made up of mysterious virus-like entities called retrotransposons, pieces of selfish DNA that appear to serve no function other than to make copies of themselves. These account for no less than 34 per cent of our genome.

All in all, the virus-like components of the human genome amount to almost half of our DNA. This would once have been dismissed as mere "junk DNA", but we now know that some of it plays a critical role in our biology. As to the origins and function of the rest, we simply do not know.