“Jesus rebuked the demon, and it came out of him, and the boy was cured from that hour.”
~Matthew 17:18

Does the Bible teach that illnesses are induced by demons?
While the boy in this story did exhibit symptoms of epilepsy, his sudden convulsions were actually being controlled by a demon. (Matt. 17:15)
Jesus’ disciples had Holy Spirit to cure the ill (Matt. 10:8)
The Bible differentiates between those who were ill and those who were demon-possessed. (Matt. 4:24)
Still, Jesus’ disciples could have expelled the demon from the boy’s body had they had sufficient faith. (Matt. 17:19,20)
In Bible history, there was someone who developed a disease as a result of a direct demonic attack.
But in the case of Job, he was being specifically targeted by Satan to try to prove points against Jehovah and against humankind. (Job 2:4-7)
It is not the standard in the Bible that illnesses or diseases are direct results of demon activity. (Rom. 5:12)
But regardless of the origin, God promises to do away with all suffering under Christ’s kingdom. (Isa. 33:24; Rev. 21:3,4)