Angels, good or bad, are the ultimate principles of the world, the ultimate foundation of the natural order. (Karl Rahner, On the Theology of Death, Herder and Herder, NY, 1961, pg. 23) Good angels will the perfection of the world because that reflects their own nature. The fallen angels, instead, will their own proud autonomy from God, willing the perfection of the universe without grace. As such, they have the power to produce physical and mental sickness. (Heinrich Schlier, Principalities and Powers in the New Testament, Herder and Herder, NY, 1961, pg. 53)
On a Sabbath, Christ encountered a woman "Who for eighteen years had been crippled by a spirit; she was bent over, completely incapable of standing erect. When Jesus saw her, he called to her and said, 'Woman, you are set free of your infirmity.' He laid his hands on her, and she at once stood up straight and glorified God." (Lk 13:11-13) Christ specifies what he meant by "crippled by a spirit" --"This daughter of Abraham whom Satan had bound for eighteen years...." (Lk13:16)
Jesus tells us that an evil spirit did this. Before trying to explain the cause, let us become aware that everything we do, think and feel is a mix of biological, cognitive, psychological, social and spiritual factors subject to demonic influence.
What caused this woman to become crippled?
Western medicine looks for "natural causes". It is not likely that a genetic mutation caused her to be born crippled. All we can say is that an evil spirit caused her deformity at the biological level of her being. We could indeed find a "natural cause" for her condition but this blinds us to its ultimate cause, that is, human subjection to suffering and death at the hands of Satan. Christ, the Son of God made man has power to banish evil spirits or the effects evil spirits upon mankind.
Here we see a father who bring his son possessed by a "mute spirit". The father tell Jesus that"Wherever it seizes him, it throws him down; he foams at the mouth, grinds his teeth, and becomes rigid." (Mk 9:18) He further tell Jesus that he had been this way since childhood and that the evil spirit often threw him into fire or water in order to kill him. Jesus said to the spirit, "Mute and deaf spirit, I command you: come out of him and never enter him again! Shouting and throwing the boy into convulsions, it came out." (Mk 9:26) This sounds like the malady we call epilepsy. The Mayo Clinic says: "Epilepsy is a neurological disorder in which disturbances is the brain cell activity cause repeated seizures." It does not appear that the child was born with a genetic defect since the father specified "since childhood", however, a later injury to the brain could have caused it. Does epilepsy involve only a natural cause? In 1970 a young woman was diagnosed with epilepsy. She wondered whether this was caused by an evil spirit, so she prayed and commanded the evil spirit to leave her, and he did. Her seizures began when she was struck on the head by a baseball. This brain injury opened the "door" by which the spirit of epilepsy entered. (spirituality.net/Michael H. Brown, Is epilepsy caused by demons?)Christ Knew that the young boy epilepsy was not due solely to natural causes but to an evil spirit bent on destroying human nature.
Demonic possession is a direct assault upon human nature.
Jesus encountered a demoniac at Gerasene. The man was naked and lived among the thumbs. This odd behavior does not mean that he is possessed by demons. The people tried to restrain with chains which he broke with superhuman strength which reveals demonic strength. Jesus ordered the evil spirit (spirits) to leave the man. A loud voice shouted, What have you to do with me, Jesus, son of the Most High God? I beg you, do not torment me! (Lk 8:28) The demoniac could not have know who Jesus was but Satan and his followers knew. Jesus ultimately banished the evil spirits into a heard of swines on the hillside which caused their death. This reveal without any doubt, the superior power of the God-Man. Christ is now"Far above every principality, authority, power, and dominion, and every name that is named not only in this age but also in the one to come." (Eph 1:22)