A demon is alive only in your mind. A demon is a very strong subconscious belief in a matter of evil. It has come into the mind either by word from another or from self imagination. It is a false belief in a power greater than God. The only power of a demon is the belief placed in it.
Demons are "cast out" by replacing the false belief with truth. Demons are often placed in the mind by others who want to exert control over you. It is a scare tactic similar to the old hellfire and brimstone threats of the past made by some preachers.
In truth and reality, the Word from God must come into the mind to cleanse the mind of the sin of believing a lie. This is the will of God, that you dwell in peace, love and joy. God has established a way for this to be your life.
The apostle John wrote of this matter in saying;
"We have known that every one who hath been begotten of God doth not sin, but he who was begotten of God doth keep himself, and the evil one doth not touch him; we have known that of God we are, and the whole world in the evil doth lie; and we have known that the Son of God is come, and hath given us a mind, that we may know Him who is true, and we are in Him who is true, in His Son Jesus Christ; this one is the true God and the life age-during! Little children, guard yourselves from the idols! Amen."
Align your will with the will of God. Learn truth from God. Learn that perfect love casts out all fear and with this, the demon. There is truth in the quote; "There is nothing to fear but fear its self." It is your fear that feeds the demon.
The physical body responds to and obeys the dictates of the mind. God has established the physical body with the ability to heal. God has also established a way for the healing of the mind and spirit.
There is nothing in the physical you can do to rid yourself of this strong belief. It is a spiritual matter. There is no stronger power than the Spirit of God. If then the Holy Spirit dwells in you, it is not possible for an evil spirit to also dwell in you.
It is the Word from God that cleanses the mind. The Word of God is truth and when believed, removes all beliefs that contradict the Word of God. Faith must be in the Word of God and no longer in an erroneous belief in a matter that is not true.