Ever heard of the black zombie myth?
The conspiracy is that the whole idea of zombies- the Hollywood kind not the voodoo kind- came from a real black plague era infection. This parasitic infection, spawned in brackish, unsanitary 14th century London, can take over the host's will and, in some cases, their consciousness to serve the organism's own agenda: feeding on and living in humans. A lot of cultural leftovers come from the black plague: children's nursery rhymes like "London Bridge" for instance. There is also some evidence that the idea of being possessed by a demon comes from these infections since, at that time, physicians couldn't know about germs. There is one demon, who stole the name of a Greek goddess, that is known as Eurynome: prince of death, and eater of corpses. A controversial theory is that parasitic fungi have been known to "animate" dead animals by perpetuating the cells' "motors" or what makes them work. The fungi can preserve them and keep them functioning even after, technically, the host is no longer considered alive by any conventional definition; hence the living dead.