The Legion Of Unclean Spirits
Demons (or "devils") are mentioned in a number of different circumstances throughout the Holy Scriptures, but always in a state of severe psychosis i.e. "any severe mental disorder in which contact with reality is lost or highly distorted." When demons infect humans, according to all references to them in the Bible, the people also become psychologically tormented. That's not to say that all mental illness is demon possession, but some very likely is.
"My name is Legion: for we are many"
The Holy Spirit in Jesus cleansed many people of unclean spirits and mental illness, whether and/or whenever they were not one in the same: "4:24 And his fame went throughout all Syria: and they brought unto him all sick people that were taken with divers diseases and torments, and those which were possessed with devils, and those which were lunatic, and those that had the palsy; and he healed them" (Matthew 4:24 KJV). Among those healed was a man in the lands east of the Sea of Galilee and Jordan River in Gadara:
"5:1 And they came over unto the other side of the sea, into the country of the Gadarenes.
5:2 And when he was come out of the ship, immediately there met him out of the tombs a man with an unclean spirit, 5:3 Who had his dwelling among the tombs; and no man could bind him, no, not with chains: 5:4 Because that he had been often bound with Fetters and chains, and the chains had been plucked asunder by him, and the fetters broken in pieces: neither could any man tame him.
5:5 And always, night and day, he was in the mountains, and in the tombs, crying, and cutting himself with stones.
5:6 But when he saw Jesus afar off, he ran and worshipped him, 5:7 And cried with a loud voice, and said, What have I to do with thee, Jesus, thou Son of the most high God? I adjure thee by God, that thou torment me not. 5:8 For he said unto him, Come out of the man, thou unclean spirit.
5:9 And he asked him, What is thy name?
And he answered, saying, My name is Legion: for we are many. 5:10 And he besought him much that he would not send them away out of the country.
5:11 Now there was there nigh unto the mountains a great herd of swine [see also Clean and Unclean] feeding. 5:12 And all the devils besought him, saying, Send us into the swine, that we may enter into them.
5:13 And forthwith Jesus gave them leave. And the unclean spirits went out, and entered into the swine: and the herd ran violently down a steep place into the sea, they were about two thousand; and were choked in the sea [see also Drowning].
5:14 And they that fed the swine fled, and told it in the city, and in the country. And they went out to see what it was that was done. 5:15 And they come to Jesus, and see him that was possessed with the devil, and had the legion, sitting, and clothed, and in his right mind: and they were afraid. 5:16 And they that saw it told them how it befell to him that was possessed with the devil, and also concerning the swine. 5:17 And they began to pray him to depart out of their coasts.
5:18 And when he was come into the ship, he that had been possessed with the devil prayed him that he might be with him. 5:19 Howbeit Jesus suffered him not, but saith unto him, Go home to thy friends, and tell them how great things the Lord hath done for thee, and hath had Compassion on thee.
5:20 And he departed, and began to publish in Decapolis how great things Jesus had done for him: and all men did marvel." (Mark 5:1-20 KJV)
Fact Finder: Does the original Biblical word from which we get "devil" actually mean someone who attacks others by means of slander? Does the original word from which we get "Satan" actually mean "accuser"?