No, I think the critical difference is not the majority (no source here, just my gut feeling) of virii and parasites are not beneficial to their hosts and the f

No, I think the critical difference is not the majority (no source here, just my gut feeling) of virii and parasites are not beneficial to their hosts and the few that are did so by chance. In nature, a virus or parasite can't spread too fast or hurt its host too much or its reproductive cycle will not be able to sustain itself. We see this in software too: a virus that is too nasty will attract too much attention and everyone will end up installing tools to detect and remove it. So, while you may find the occasional symbiosis in nature between host and parasite, there is little reason for that to happen in the software world. It's far easier, safer, and potentially more lucrative to fly under the radar. For example, once your software infects a computer and you get access to their bank account, you only take $7.69 out once every couple months or so instead of draining it in one go.