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.
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"?