The first step to answer this question is to try and establish what demon possession actually entailed.
The short answer is that people of Jesus’ day understood demons to be the cause of otherwise unexplainable ailments (a belief they had inherited from other cultures). For a more detailed analysis please see the answer to this question: [...]
Answers about illness
See also What is the difference between “unclean spirit” and “demon”?
Why does OT and NT teaching on demons appear to differ?
It is undeniable that there is a difference between the way the OT and NT describe illness and disability:
Exodus 4:11 Then the Lord said to [Moses], “Who has made man’s mouth? Who makes him mute, [...]
