As i have usually heard it in lecture, viruses are 'obligate intracellular parasites', which depend explicitly on cellular nuclei for reproduction. My understanding is that the reproductive action of a virus requires the destruction of the host cell. This spells doom for single-celled organisms, although not necessarily for large populations of them, such as biofilms, or in our case, as multicellular organisms.
However i've always had a little voice of doubt in the back of my mind on this point, I've wondered whether there was any kind of 'benign' virus which was capable of interacting with it's host without being so destructive (and therefore high profile).
Usually when i bring up 'positive' roles for viruses, i hear about viral transduction, such as in laboratory settings, but also as implicated in horizontal transfer events 'in the wild'. Nevertheless my understanding of viral transduction is that it requires a pathogenic stage in order to first acquire genes from the donor organism before transmitting them to the recipient. For the sake of clarity, in this sense 'pathogenic' refers not just to causing disease in humans or other animals(and plants also, i suppose), but also in bacteria, which i guess scientists dont usually consider capable of 'getting sick'.
At any rate, such a virus would need to enter the cell, uncoat, hijack enzymes to replicate new viral dna(or rna, as the case may be), as well as ribosomes to produce the appropriate capsids and then exit the cell without causing more harm than the host cell can repair, certainly without lysing the cell. This sounds quite exceptional, barring a very clever solution of course, which nature has provided before.
The point of this post is to ask any resident experts whether there is evidence of such a thing existing in nature. If not thanks anyway.
