>>12224479because viral agents lack the necessary "bio bandwidth" to do so. A virus's entire survival strategy is based on parasitism since it can not reproduce on it's own, it NEEDS to subvert the hosts biochemical mechanisms and insert itself in to the hosts DNA to reproduce and can only finalize it's reproductive cycle upon the killing the host cell to release it's new copies.
Compare this with bacteria which regularly form mutualistic relationships with all kinds of multicellular organisms. Bacteria are self replicating and can exist on their own so long as there certain environmental requirements are met. Bacteria form relationships with other organisms to improve their own chances survival by improving the chances of the other organism, viral agents can't do that by their very nature.
But for the sake of argument if you wanted to make a virus that was beneficial to humans you could only do it in one way baring radically altering it's structure to the point of no longer being a virus and that is defect cleanup. You would have to alter the virus in such a way that it only attacks defective cells, like cancer cells. The problem with this is that it goes against the laws of nature you would have to make a vius who's survival strategy relies on a condition it exists to prevent, ie cancer spreading though the body so it could consume the correct cells and continue existing.
