My feeling is that these parasites are not just "deep" but actually not even physically manifest until they come up for clearing. And that parasites, blockages, karma, whatever-- are spread out over all our incarnations over several millions of years.
dowsing says strongly yes, entities= parasites and parasites are doorways for them to get in body or mind or emotions or astral field. which explains layering and piggybacking I think.
also says strongly yes they(critters) are able to jump into other parts of the DNA template((our) other simultaneous incarnations in space/time, and shift around to hide or interfere during periods of significance. This explains psychic attaching, and why things run "so deep."
I'm pondering lots and lots today... :)