When the embryo is formed it appears that what does happen from the get go is that each cell is provided with its own unique astral body or super computer processor. At a later point in development an additional spirit body containing the pattern of an individual soul attaches itself and effectively takes charge in terms of guiding additional development. This is not immediate on conception but is also flexible in terms of joining. thus we have identified two critical components of the human spirit body.
This also informs how the spirit body known as the green man can join hundreds of trees in an ongoing calving of aspects and recovery of aspects.
We also know that it appears that non human spirit bodies can at least attach themselves without been able to exercise significant control over the astral body or at least that is what appears to be likely.
Thus evil spirits can enter this scenario and must be guarded against usually by rigorous thought. Evidence abounds, but in most cases their intent is clearly contrary to the victim's own intentions. The greatest risk comes for those listening to their spirit body getting misinformation and must be guarded against.
Mind Virus Wetiko: Collective Shadow of Humanity