IF A DEMON IS NOT A MATERIAL BEING, HOW CAN A DEMON BE "IN" A BODY?
Two simple analogies help explain this phenomenon based upon our understanding of angelic natures.
The first is the analogy of a container. A spiritual being cannot be technically contained "in" a space because only a materiel being can take up space or seemingly be confined to space. Rather, it is better to say that the demon contains the space within its malicious mind and will. It is present there by its innate powers to think about its victim and the will to harm him. Peter Kreeft, citing the theology of St. Thomas, describes the demonic presence in a body or place like a stage upon which the demon acts. The stage "contains" the play and so also does the demonic will "contain" the action of his persecution, but he is not contained by the body. The same can be said of angels for good, of course.
The second analogy is that of light. Sunlight can be seen everywhere on a bright day, sent to us in rays from an immense burning star that is 93 million miles away from the surface of the earth. In a sense, the world is contained "in" the sunlight, not the other way around, because the sun is coming to it from "beyond" and encompassing it. Yet, someone may take a magnifying glass and willfully focus the power and intensity of the light on some definite object within that overall scenery where the sun shines. If he focuses it long enough on a material surface, the intense focused beam may penetrate the object with great harm. That magnified light beam and its destructive power is similar to the intensity of the destructive demonic mind and will on his object.
Fr. Thomas J. Euteneuer, "Exorcism and the Church Militant" page 35.