Malware has bugs. In fact some viruses have done far more damage than the author intended due to bugs. There was a substantially effective virus years ago that

Malware has bugs. In fact some viruses have done far more damage than the author intended due to bugs. There was a substantially effective virus years ago that made it around the world in 90 minutes, and it turns out a bug in its networking code caused it to spread half as fast as it should have. Meaning it should have been everywhere in 45 minutes. You can still do a lot of damage without hitting every machine in existence.