They cannot penetrate a human who has a strong defensive guard against them. So they look for men and women who are vulnerable. People who are physically and mentally weakened by sickness, disease, abuse, etc, are the very targets that unclean spirits most want to find. Then, in short order, the victim has Double Trouble. Not only must he or she combat the physical problems, but now they must recognize the works of Satan as well, and resist that too.
Spiritual oppression can be unbelievably horrific.
Demon oppression and demon possession can wreak havoc in a person's life. Both needs must be addressed. If there is a physical weakness or ailment, that must be addressed. If there is an addiction, that addiction must be broken. Substance abuse must be terminated.