As seen in...
Michael Crichton's novel Prey, The PS2 game Nano Breaker What are they?
Nanobots are a technology that science apparently engineered to make you terrified of the future. We're talking about microscopic, self-replicating robots that can invisbily build--or destroy--anything. Vast sums of money are being poured into nanotechnology. Sure, at some level scientists know nanobots will destroy mankind. They just can't resist seeing how it happens. How it can result in zombies:
Scientists have already created a nano-cyborg, by fusing a tiny silicone chip to a virus. The first thing they found out is these cyborgs can still operate for up to a month after the death of the host. Notice how nano scientists went right for zombification, even at this early stage. They know where the horror is.
According to studies, within a decade they'll have nanobots that can crawl inside your brain and set up neural connections to replace damaged ones. That's right; the nanobots will be able to rewire your thoughts. What could possibly go wrong? Chances this could cause a zombie apocalypse:
Do the math, people.
Some day there will be nanobots in your brain. Those nanobots will be programmed to keep functioning after you die. They can form their own neural pathways, meaning they can use your brain to keep operating your limbs after you've deceased and, presumably, right up until you rot to pieces in mid-stride.
The nanobots will be programmed to self-replicate, and the death of the host will mean the end of the nanobots. To preserve themselves, they'd need to transfer to a new host. Therefore, the last act of the nanobot zombie would be to bite a hole in a healthy victim, letting the nanobots steam in and set up camp in the new host. Once in, they can shut down the part of the brain that resists (the cortex) and leave the brain stem intact. They will have added a new member to the unholy army of the undead.
Now, it should be more than clear by this point that our goal is to be responsible researchers. We don't want to create a panic here. All we're saying is that on an actual day on the actual calendar in the future, runaway microscopic nanobots will end civilization by flooding the planet with the cannabalistic undead.