What the Bible says about Lunatic
(From Forerunner Commentary)
Christ interrupts the father's appeal to ask about the duration of the child's affliction. The man answers, "From childhood," that is, since he had been a small child. The boy was still young, as the Greek word translated "child" in Mark 9:24 implies, but the sense is that the affliction had lasted many months or years. Sin, too, has been with us since we were children (Psalm 58:3; Isaiah 48:8).
The boy's illness—"epilepsy" (NKJV) or "lunatic" (KJV)—was then regarded as a disgraceful disease. Some thought it fell on those who had sinned against the moon (luna, "moon") and changes in the moon were said to govern the period of epileptic seizures. Epilepsy was also connected with demon possession, but we know today that being epileptic does not mean that a person is demon possessed.
The boy's convulsions are sudden and last an incredibly long time, for the evil spirit "departs from him with great difficulty." The demon's frenzy becomes more violent in Jesus' presence: The boy falls on the ground, rolls around, and foams at the mouth. Many who have given themselves to sin could be said to act crazily and irrationally. They may be highly respected people in society, but they do not act sensibly when it comes to life's most important matters. They have no interest in salvation, God's truth, or His Kingdom, justifying all sorts of evil. Their choices to do self-damaging acts shows the craziness of their thinking.
Martin G. Collins
The Miracles of Jesus Christ: Exorcising a Young Boy (Part Two)