How can we have a world that is inhabited only by zombies and yet continues to be populated?
For the purposes of this question, I envisage the type of zombie that has been infected by a virus, is clinically dead, yet still able to move around, is compelled to seek out non-infected living people, and is gradually rotting away.
Notes: If the head is completely separated from the body then they are no longer active. They require at least some remnant of muscle tissue to move around. Once all the flesh has rotted off they are simply inanimate skeletons.
Zombies can be considered to retain a degree of intelligence (perhaps half their original IQ) but are driven by a lust for fresh human flesh even though there is no longer any available.
How can I explain a planet completely populated by the above type of zombie for thousands of years where all normal people died out long ago. (There are no pockets of 'normals' anywhere on the planet whether in captivity or otherwise).
There is no magic in this world.