As we suffer through the end of cold and flu season here in the great white north (Canada) I'm well aware of the effects of viruses. I've often wondered why it is that viruses exist in the first place.
Some may say this question is as naive as asking why do mosquitoes exist? Viruses, after all, can be thought of as simple cellular parasites, using our cells to reproduce and spread. Just the same mosquitoes don't require an intimate knowledge of the workings of our cells, indeed the very core of our cells, to function. Given that viruses are able to exploit our cellular processes suggests that they may themselves be part of a cellular process.
If you consider sperm, they are essentially packages of DNA with an outboard motor. It isn't a huge stretch of the imagination to imagine that viruses may be a method of information exchange. In the bacteria world, genomic information exchange (aka sex) occurs via the exchange of 'plasmids'. Plasmids are circular bands of DNA which can readily be exchanged and recombined among bacteria. Plasmids are typically not parasitic but more symbiotic and convey useful traits such as antibiotic resistance. Viruses can also behave in this way. Endoviruses, viruses which incorporate themselves back into their hosts DNA, could be used as a method of information exchange and transfer.
I've often considered an effective method for wiping human viruses off the planet in one felled swoop. Viruses hijack cellular factories called ribosomes which manufacture proteins for us. When infected, our ribosomes execute the viral code which in turn produces more viruses. To disable all human viruses wouldn't require much effort at all. Viruses and ribosomes speak the language of RNA which is a derivative of DNA. Ribosomes currently have no good way of differentiating between human RNA and viral RNA. (As an aside, RNA sequences that contain siRNA (small interfering RNA) segments are a good clue that the sequence is viral. This causes the cell to act to destroy such sequences. However, we all still get sick, thus it clearly isn't 100% effective.) To effectively wipe out all human viruses all we would need do is add some well known key to all human RNA sequences. The ribosome would then reject all sequences missing this key. This would instantaneously wipe out all human viruses (which would lack this key).
So why don't we do this? First it's beyond our current technology to alter our DNA in such a fashion, but it won't be long before it's within our grasp. The question is: would we want to? Elements of our genome already act like viruses. Genes called transposons effectively jump from one part to another. Disabling viruses may somehow preclude a vital source of information exchange. In our oceans, bacteria 'suffering' from viral infections work to produce oxygen and sequester carbon dioxide. Without this viral 'infection' it's likely life as we know it couldn't exist on earth.
So the next time you're run down with a virus, consider that you're merely the victim of a side effect of a process essential to life on earth. Perhaps this is why we say "Bless you" when someone sneezes?