Let's see, so far as I can tell all undead arise from one of four sources:
1. Disease: Modern conceptions of zombies fall into this category, as do ghouls. Vampires can be and have been argued to fall into this category as well.
2. Divine Intervention: Vampirism as "the curse of Cain" falls into this category. Similar to undead created by magic, the difference being the entity wielding the power resulting in an undead creature.
3. Created by magic: Typical RPG "created undead" of course fall into this category, as do traditional voodoo zombies.
4. Created by events: "Hauntings", undead who are fettered by location, would be an example of this.
There's no particular reason I can spot to confine a given "species" of undead (say, zombies) to a particular method of creation (say, disease) any more than to tie it to a particular base creature type (say, humans). If vampirism is a disease, are elves subject to it in the same way as humans? Are they subject to it at all? Or perhaps does it have different symptoms and get called by a different name when it happens to an elf? Even if it is a disease now, that doesn't necessarily mean it didn't start as a curse, whether from a mortal or Divine source, originally, does it? Is the RM Plague spell a disease, a spell or a curse? Answer: Yes.
Okay, but one of the symptoms is that it makes the victim "immortal"... so in theory there could be one or more of the original targets still around, under the effects of a curse, mixed into a population of who knows how many who are under the effects of a disease with functionally identical symptoms.
In most ways I'd think any "type" (eg vampire) could come about by any of the methods above, within obvious limits. Most things as powerful as Gods won't bother creating something as generally useless as a zombie, an undead produced by disease presumably means there is a corporeal body the disease inhabits, powerful undead created by magic posits a necromancer more powerful still than any of his creations, extraordinary undead created by events must have been created by extraordinary events, etc.
Thoughts?