The answer how unclean spirits are united with human souls.
Serenus: It is no wonder that spirit can be imperceptibly joined with spirit, and exercise an unseen power of persuasion toward what is allowed to it. For there is between them (just as between men) some sort of similarity and kinship of substance, since the description which is given of the nature of the soul, applies equally well to their substance. But it is impossible for spirits to be implanted in spirits inwardly or united with them in such a way that one can hold the other; for this is the true prerogative of Deity alone, which is the only simple and incorporeal nature.
Source: Conferences (New Advent)