Plague

Once bitten, the newly infected were filled with an insatiable hunger, for the brains of the living, millions perished, but only to rise again, as legions of plagued undead.

To protect the living, the grand barrier was built,  a 100 foot wall encircling London, then excavation began on the royal canal, a vast mote 30 fathoms deep,  surrounding both the cities and it’s walls,  the land between the two fortifications became known as the In-Between.

Early Stages
"'A newly infected Zombie is almost impossible to detect, until they have ingested their first human brains, at which point transformation accelerates with every subsequent kill.' -Mr. Darcy"

Substantial Kills
It is a truth universally acknowledged, that a zombie in possession of brains will be in more want of brains, never was this truth more plain than the recent attacks at Netherfield Park, in which an entire household was slaughtered by a horde of the living dead during a party.