A question about the love of god

[John V]

Matt 17
When they came to the crowd, a man came up to Jesus, falling on his knees before Him and saying, 15 “[f]Lord, have mercy on my son, for he is a [g]lunatic and is very ill; for he often falls into the fire and often into the water. 16 I brought him to Your disciples, and they could not cure him.” 17 And Jesus answered and said, “You unbelieving and perverted generation, how long shall I be with you? How long shall I put up with you? Bring him here to Me.” 18 And Jesus rebuked him, and the demon came out of him, and the boy was cured [h]at once.

What disease does the boy have, and what is its cause?