Some core telltale signs of a demonic attack (or demonic oppression) are that demons compel and enslave the person. It could be a compulsion to use drugs, drink alcohol, watch pornography, masturbate, attack someone (verbally, physically, or other), dabble with the occult, etc. Basically, anything that involves sin. It is possible for a person to be enslaved to a demon of anger, bitterness, hatred, or wrath. Not letting go of anger or having hatred, animosity, or bitterness towards someone can very well be the enemy manipulating the emotions of a person to be enslaved to such negative feelings.

But something that I think can easily be overlooked is that demons can cause physical sickness or infirmity. Research shows that lingering feelings of bitterness can affect you physically:

We all experience a bout of sour grapes from time to time, but experts are warning that being too bitter too often can actually make you ill.

Researchers who have examined the relationship between failure, bitterness and quality of life found that blaming others can lead to physical disease.

‘Persistent bitterness may result in global feelings of anger and hostility that, when strong enough, could affect a person’s physical health,’ says Carsten Wrosch, a professor in Montreal’s Concordia University Department of Psychology and a member of the Centre for Research in Human Development.

Source

Unforgiveness is classified in medical books as a disease. According to Dr. Steven Standiford, chief of surgery at the Cancer Treatment Centers of America, refusing to forgive makes people sick and keeps them that way.

With that in mind, forgiveness therapy is now being used to help treat diseases, such as cancer.

“It’s important to treat emotional wounds or disorders because they really can hinder someone’s reactions to the treatments, even someone’s willingness to pursue treatment,” Standiford explained.

Of all cancer patients, 61 percent have forgiveness issues, and of those, more than half are severe, according to research by Dr. Michael Barry, a pastor and the author of the book, The Forgiveness Project.

Source

I don’t doubt that a lot of physical sicknesses are spiritually caused. However, I am not going to be dogmatic and declare all physical sickness is the consequence of a demonic attack. But with my experience in deliverance, I have come to know that harboring feelings of bitterness or forgiveness or deliverance can open doors for demons to come into a person’s life. The established link in the medical field with bitterness and unwillingness to forgive with physical sickness bolsters the idea, in my view, that it could very well be an affliction by a demon, particularly a spirit of infirmity.

I believe Hollywood has greatly distorted our ideas on telltale symptoms of demonic affliction. Not everyone who has demons acts like the little girl in the film The Exorcist (which by the way the people involved with the film got messages via the use of the Ouija board) with her head-turning and crawling up the wall, although the Gadarenes demonic (Luke 8:26-39) could fit this description. There are cases in the Bible where people had demons, but if we would encounter them today, it would likely never cross our minds the person was afflicted or inflicted by them. For example, the blind and mute man (Matthew 12:22) and the woman with a bent back (Luke 13:11).

In Matthew 18, Jesus gives us this parable about our need to forgive and the consequences of not forgiving others:

^“23 Therefore is the kingdom of heaven likened unto a certain king, which would take account of his servants.²⁴ And when he had begun to reckon, one was brought unto him, which owed him ten thousand talents.²⁵ But forasmuch as he had not to pay, his lord commanded him to be sold, and his wife, and children, and all that he had, and payment to be made.²⁶ The servant therefore fell down, and worshipped him, saying, Lord, have patience with me, and I will pay thee all.²⁷ Then the lord of that servant was moved with compassion, and loosed him, and forgave him the debt.²⁸ But the same servant went out, and found one of his fellowservants, which owed him an hundred pence: and he laid hands on him, and took him by the throat, saying, Pay me that thou owest.²⁹ And his fellowservant fell down at his feet, and besought him, saying, Have patience with me, and I will pay thee all.³⁰ And he would not: but went and cast him into prison, till he should pay the debt.³¹ So when his fellowservants saw what was done, they were very sorry, and came and told unto their lord all that was done.³² Then his lord, after that he had called him, said unto him, O thou wicked servant, I forgave thee all that debt, because thou desiredst me:³³ Shouldest not thou also have had compassion on thy fellowservant, even as I had pity on thee?³⁴ And his lord was wroth, and delivered him to the tormentors, till he should pay all that was due unto him.³⁵ So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses.”

Who are these tormentors? These tormentors in my view are demonic spirits to which a person opened the doors to because of a lack of forgiveness.

The scriptures also read that bitterness can defile a person: