Mark 9:14 And when he came to his disciples, he saw a great multitude about them, and the scribes questioning with them.
The scribes were responsible for preserving the word of God and were often in opposition to the moving of God through Jesus and his disciples, always questioning their authority.
Mark 11:28 And say unto him, By what authority doest thou these things? and who gave thee this authority to do these things?
Mark 9:15 And straightway all the people, when they beheld him, were greatly amazed, and running to him saluted him.
Why were they amazed when they saw Jesus? Because he still had the glow he had on the mountain when he appeared with Moses and Elijah.
Mark 9:3 And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them.
Possibly his face also glowed as did Moses when he returned from the mountain after being with God.
Exo 34:29 And it came to pass, when Moses came down from mount Sinai with the two tables of testimony in Moses' hand, when he came down from the mount, that Moses wist not that the skin of his face shone while he talked with him.
Exo 34:30 And when Aaron and all the children of Israel saw Moses, behold, the skin of his face shone; and they were afraid to come nigh him.
Exo 34:35 And the children of Israel saw the face of Moses, that the skin of Moses' face shone: and Moses put the veil upon his face again, until he went in to speak with him(God).
This is referred to by Paul, concerning Israel being unable to see the glory of God and to understand spiritual things.
2 Cor 3:13 And not as Moses, which put a veil over his face, that the children of Israel could not stedfastly look to the end of that which is abolished:
2 Cor 3:14 But their minds were blinded: for until this day remaineth the same veil untaken away in the reading of the old testament; which veil is done away in Christ.
2 Cor 3:15 But even unto this day, when Moses is read, the veil is upon their heart.
2 Cor 3:16 Nevertheless when it shall turn to the Lord, the veil shall be taken away.
2 Cor 3:17 Now the Lord is thatSpirit: and where the Spirit of the Lord is, there is liberty.
The scribes had the veil upon their hearts, concerning Jesus.
Mark 9:16 And he asked the scribes, What question ye with them?
Mark 9:17 And one of the multitude answered and said, Master, I have brought unto thee my son, which hath a dumb spirit;
Mark 9:18 And wheresoever he taketh him, he teareth him: and he foameth, and gnasheth with his teeth, and pineth away: and I spake to thy disciples that they should cast him out; and they could not.
His disciples were unable to cast the spirit out.
Mark 9:19 He answereth him, and saith, O faithless generation, how long shall I be with you? how long shall I suffer you? bring him unto me.
Jesus is able, we must have faith in God through Jesus. This ailment sounds like epilepsy. According to the story, it was caused by a bad spirit. He was said also to have had a dumb spirit. Probably today's doctors would say it was something in the brain. Most today of the world do not see the spirit world. The cause was a bad spirit. Jesus is Lord of all.