21. Computer Chip Implant
There are a lot of smaller devices today, many working off of chips that are no bigger than your smallest fingernail. Funny enough, they are actually getting smaller by the day. Imagine for a second that you could actually use a chip implant in someone and have access to their body or brain. If you have control of their central nervous system, you could force them to do things they do not want to do.
The person could consciously be doing this or you could find a way to put them to sleep. Meanwhile, if you only have access to their brain, you could make them basically go crazy and do the things you tell them to do. Like an inner voice inside their head, similar to a God-like figure. Several mass shooters in the past have claimed the voice of God told them to do what they did. Perhaps a real zombie apocalypse could occur in a similar way.