Like a self-replicating contagious vaccine. I've always wondered about this in the context of computing. Why no one creates "virus" that infect a machine, fix

Like a self-replicating contagious vaccine. I've always wondered about this in the context of computing. Why no one creates "virus" that infect a machine, fix the exploit and spread to others using the same exploit used by the original virus. Like an immune system for the network.