Doctor Downway created a virus. He was experimenting with nanotechnology for the U.S. government in a top secret lab. He had made great progress but one of the limits of nanotechnology was keeping those little guys under control.
See, for nanites to work, you either have to manufacture octillions of them or you have to make them self replicating. It takes too much time and money to manufacture octillions so most scientists have chosen self replicating and letting the nanites themselves create more. That's where the risk comes in. How do you get nanites to self replicate into the octillions and then stop before they take over the entire world.
Doctor Downway had found the answer. The first nanite would have 100 tags. When that bot created a replicate, it would destroy one of its own tag and create the replicate to have one less tag as well. Then there would be two nanites each having 99 tags. This would repeat leaving four nanites having 98 tags each. On and on this would go until the nanites ran out of tags.Then there would be more than 1,000,000,000,000,000,000,000,000,000,000 nanites with zero tags each. The nanites would do their job and then self destruct.
Another problem with nanites is determining what their job would be. nanites could do just about anything. They were especially good at transforming one kind of bodily tissue into another. They could be used as a weapon or they could be used to heal. They could even be used to improve soldiers. If a soldier is obese, the nanites can transform all that fat into muscle. But the government wasn't sure what they wanted to do with the nanites so they let Doctor Downway get creative.