Because viruses mutate? Even something that is harmless in the beginning might become lethal later on. And even a "harmless" virus might kill immuno-compromised

Because viruses mutate? Even something that is harmless in the beginning might become lethal later on. And even a "harmless" virus might kill immuno-compromised or weakened individuals. Your idea reminds me of using a computer worm to patch backdoors. I'm sure I've read of this before.