User blog comment:Zathronas/Speculative Author - RWBY Relativity/@comment-25396609-20160225025456/@comment-25396609-20160225034911

The WoR explaining aura and dust were both provided by the single most unreliable narrator of all time. Neither of them was scientific, either. Just vague explanation.

I will openly admit my theory on grimm is, at best, tenuous, but as has been said in show and in WOR their origin is totally unknown. All we know is that they are antithetical to humans.

I also admit the more likely explanation is that they are manufactured (probably by Salem or someone similar) and were programmed to hunt down mankind specifically.