@KoiwiGal - There's actually a polymorphic virus like that, which I think of as one of the most famous of computer viruses. It hasn't done anything yet, but it could one day.
I can't remember all the details about what it's called, but it's an extremely successful computer virus that has infected a large majority of computers. A team was tracking it and they eventually realized it was polymorphic and very difficult to pin down. The person who made it went to a lot of trouble to hide it so it will probably be used for something eventually.
But, the point is, you've probably got it on your computer right now and don't even know it. And your workplace probably has it on all their computers as well. It's a scary notion.