Messing about with someone's head is creepy. Abusing technology to do so only enhances the creep-factor. So, let's max out the creepiness with wee little beasties. What could be more creepy than little bugses crawling around in your brain.
Grey Worms are microscopic parasites that live within the cerebrums of chordates. The larger the brain & higher the body temperature, the better host that animal makes. Humans are ideal, but large herbivores work fine, too. Grew Worms are able to tear through the blood vessels and enter the brain. As they eat intra-cellular matter--and are famished from their journey--they consume what would otherwise be a minor stroke while the rip seals. Grey Worms reproduce asexually, and stay within the host until its death. From there, they encyst themselves and are eaten when a creature accidentally bites the soil while grazing, or for humans, when food is fertilised with contaminated tissue and not washed properly.
More diabolically, some shadowy organisations have begun injecting Grey Worms directly into their victims. Even a single cyst placed in the victim's food, or under the skin with even a scratch, will have Grey Worms reproducing & altering the brain within the hour.
Once they are inside the brain, the Grey Worm makes itself right at home. A fairly passive parasite, it does little direct damage to the brain. No neural cells are harmed, and few resources are consumed. As the process of encystment & spreading to a new host is both difficult and risky, the Grey Worm's survival habit is to instead interfere with the host as little as possible.
What the Grey Worm does is move things around a bit, like rearranging furniture. The host is not impaired, but personality changes are evident. These are subtle changes, and not usually noticed in cattle or most other hosts, but in humans, the changes to memories and outlook can be dramatic. As the overall functioning of the brain is unaffected, the conscious mind attempts to make sense of the new patterns of thought, and what it thinks it remembers. Only in rare cases are major skills--such as language or walking--hampered, but changes such as gaining religious fervour or losing loyalty to one's nation are surprisingly common. Minor details--nevertheless quite troublesome to those familiar with the host prior to infection--are also very common, such as food preference or manner of dress.
The Grey Worm infestation is there for the long-term. There are no ways to remove them without also killing the host. Most of the changes in personality & memory are stable shortly after infection, however in some cases further alterations can occur years later, usually when something (such as a bad flu or a really drunken bender) stir the worms up & cause them to again reorder the neural pathways.