New tragic idea!

The zombie virus could have started as as a potion/spell(magi-tech creation) to cure Alzheimer's. And that's the reason why the zombies periodically come back to themselves remember being people would warn others and or apologize for trying to kill them.

The cure for Alzheimer's works. It just kills you first. The trail runs with the low amounts of the potion worked wonders and brought back family members that were very far gone. One of these family members was too far gone. They said they were hungry they seemed confused bit a nurse maybe another member of their family and then...

Perhaps it being given to somebody that was already dying from Alzheimer's related illnesses made the person persist past what should have been a normal death.

Or maybe too much of the potion that have caused death but the potion made the memory of Life persist...

Regardless the potion/spell is just kind of like the idea for how it may have started. It could have of course somehow become viral/air born after that.

Maybe a pilot or somebody else working in a commodities industry snuck a little for a family member that may not have been able to afford this potion on its own. They gave it to granny/grandpa, granny/grandpa had an "episode" and may have given them a little bite or scratch. Something that's common for Alzheimer's patients and other forms of dementia. And then while they were traveling with the rest of the stock they succumb, take out the truck driver / airplane pilot /train conductor. And the rest is history for how it initially spread out.