You can kill em, crush 'em, bomb 'em and burn 'em, but getting rid of Orks is never that easy. Even as they die they give off spores that are carried upon the winds like pollen. These occasionally fall in the wild areas where their adversaries might ignore. In these uncivilised areas the spores will grow into Orks of a sort, but with no contact with the rest of Orky society. The Feral cousins of the Greenskin menace don't have access to the deadly technology of the common Space Ork and so learn to be fearsome hunters, if a little primitive, even by Ork standards.