system
Given the following post, respond as if you're commenting on Reddit:
human
Why have viruses not evolved to the point of being beneficial to their hosts, rather than usually being harmful?

Most viruses when they infect their hosts are going to be attacked and defeated by our immune systems.  If a viruses goal is to ultimately replicate, wouldn't they be more successful if they helped strengthen their hosts in some way?  Imagine an std that resulted in better vision,  Etc.
gpt
Even in theory it seems difficult to envisage how a virus could benefit a host. While bacteria can form mutualistic relationships with other species, they are able to do this because they can offer unique metabolic reactions to their host. Viruses however do not perform any function other than replication, and any other genes they have are designed to facilitate their access to the replicative machinery of other cells. So unless you could envision a function which would be improved through a replication process it is difficult to see how a virus could offer a simple and direct benefit to the host.

The other problem here is an evolutionary one. There are two evolutionary barriers to your proposition. The first is in the virus; in order to take on extra functions the virus would almost certainly have to expand its genome. However, this would almost certainly make it less fit relative to other viruses of it's type. Larger genomes take longer to replicate, are more difficult to insert and pack, and require more resources from the host cell. Viruses are heavily optimised to reproduce in large numbers, fast; so any change which reduces their ability to replicate fast, without compensating in some way (better immune evasion perhaps) will result in a fitness loss, and would be gradually selected out of the population of viral types.

The second evolutionary barrier is in the host. In order for the host immune system not to systematically kill the 'beneficial' virus, there would have to be a complementary set of chance mutations which altered the immune system in some way such that it ignored these particular viruses; while attacking every other type with perfect fidelity. Then these hosts would have to breed, and would have to have greater reproductive success as a result of their symbiosis with the virus such that the virus-friendly host type spreads. In reality this is unlikely, and would take a significant period of time. The main reason it is unlikely is that in the short term there is no benefit to someone in having a selective immune system of this type; especially since if someone had one, they would be vulnerable to any pathogenic virus which could mimic the 'beneficial' type.