Thinking more about your above comment, Guttermouth, the first time I realized that natural selection of species doesn't guarantee that we live much past reproductive age, just long enough to see the kids on their way, was chilling. Yet nature is kind enough to give us twice that long usually. Kind of like viruses that could as you say, kill off individuals when they're done with us, but usually don't. Peaceful symbiosis is likely more tenable for viruses also than outright slaughter.