It is a fact that all robots default settings is to crush, kill and destroy, but what you are suggesting is a nonobot functioning like a virus of sorts.
What senario are you proposing? When would a beneficial nonobot revert to it's primary derectives of crush, kill and destroy?
The nanobots don't need to smart at all. They do the job they are designed for and then stop working. They don't need to reproduce, really. If you need more get another dose.
Science fact? Science fiction, more like.
This seems to boil down to a Frankenstein's monster effect where we fear what we build will one day default to crush, kill and destroy.
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