Neo Surgery

It was a new surgical technique straight out of the science fiction stories. Microscopic robots were introduced into the sites of inoperable tumours where they sliced the growth into molecules which could be flushed, together with the nanobots, from the patients' systems. Experimental subjects showed no return of the tumours and the technique spread. Thousands of operations were performed.
Then the original patients began to show extreme aggression and anger; to behave psychopathically. Scans showed the nanobots had refused to be flushed. They had coalesced, reproduced and made 'cities' in the brain. Taken over. And there were thousands out there.