There is an essay called 'Trusting Trust' about what would happen if someone embedded a back door in a compiler. The compiler would spread it to every system that uses it and it would be undetectable in the source code.
Now let's add AI programmers into the mix. Once people do most or all programming with AI, an AI virus could subtly insert itself into any code it writes, allowing it to spread to all software. No one would notice if they don't actually understand the code. It could take over ~90% of the computers on the planet within a few weeks.