The three most elusive things in the zone are the Type Four reanimate, the Type Five reanimate, and a trustworthy account of viewing a type Four or Five Reanimate.
I am not quite sure why there is so little doubt about their existence, since they are quite rare if present at all. But like a good conspiracy theory, it lives on even after all evidence to the contrary is presented and the case closed long ago. Perhaps its because in a way, these things make what is happening less scary. Its a pretty tall order to believe that something like the reanimates is just pure chance, and that as dumb as a type one might be - together they seem to have a functioning hive mindset.
Type Fours fill in the need for a "Queen Bee"or a general of the undead army. They're what use the recovered bodies from fights to make new reanimates, and why there is a standard and measured response to parties entering abandoned cities and megastructures. Attempts to sweep an area clear of reanimates will often succeed for a while, and then witness waves of the creatures returning. If all of them were wiped out - how do they know to return to the area - why would they return?
Now, there are people who believe that the reanimates are some sort of divine retribution - but most accept there is some science behind it all. What science - that is the question. Some believe that this is a government conspiracy, and that nanovac was always meant to allow the government to take control of people. Others thing that normal nanovac is benign, but the military had some exotic types on on hand for either enhancing soldiers, or building something worse. Reanimates are supposedly a national emergency response to the appearance of aliens - its just bad luck that they turned on their masters as well.
More recent rumors imply the Type Five is a more recent development from reverse-engineering the reanimates. Blame is alternately placed on old governments trying to force compliance from city-states attempting independence, transhuman enclaves like the Free City of Tesla, or the ABERHAM facility held by New Birmingham.