St. Matthew 17:14-18: And when they had come to the multitude, a man came to Him, kneeling down to Him and saying, “Lord, have mercy on my son, for he is an epileptic and suffers severely; for he often falls into the fire and often into the water. So I brought him to Your disciples, but they could not cure him.”
Then Jesus answered and said, “O faithless and perverse generation, how long shall I be with you? How long shall I bear with you? Bring him here to Me.” And Jesus rebuked the demon, and it came out of him; and the child was cured from that very hour.
It is common for people to believe that because we can cure physical problems with physical science that therefore it is superstition to believe maladies are caused by demons. This logic does not hold. Man was made a unity of body and soul. The soul was not created first, but the body. God formed the body from the dust, and then He breathed into him the breath of life. Likewise Christ did not raise from the dead as only a soul, but with His body. He will return again in glory and raise all the dead in their bodies, some departing to condemnation and some to eternal life.
So it is not that physical cures prove an absence of spiritual causes, but that physical cures simply have an effect. Doctors provide a great service, and their talent is God-given.
Sirach 38:1-4: Honour a physician with the honour due unto him for the uses which ye may have of him: for the Lord hath created him. For of the most High cometh healing, and he shall receive honour of the king. The skill of the physician shall lift up his head: and in the sight of great men he shall be in admiration. The Lord hath created medicines out of the earth; and he that is wise will not abhor them.
But when Christ comes He heals spiritually. Many times demons are cast out, for they are the culprit at work. Perhaps the boy in this pericope would have been helped by anti-epileptic medication, perhaps not. Likewise, not every epileptic is necessarily demon-possessed or demon-afflicted. But either way the affliction points us toward Christ, who heals with a spiritual power, which is His creative power that has effect over the body because it is from this power that the body comes.
Often we pray for healing. Sometimes it doesn't happen as quick as we like, or at all. We might say to ourselves what Christ said, "O faithless and perverse generation!" It's okay to accuse ourselves so that we may recognize our faults, weaknesses, and limitations - and see ourselves as we are and as God knows us to be. But it is also better to see in the praying and waiting and struggling (we ought to struggle forward if we are going to bother praying, but with attention paid to the direction God is leading) the goodness of God. If He does not answer, then it may just be that our Good God has left us with something we don't like but that is good for us.
We must remember to believe that Christ is good. He is vigilant and faithful. He knows when to test us, when to push us, and when it's the right time to answer our pleas. We see in the Gospels that He is good, and we see that sometimes He seems to put us off (like the woman He called a little dog or Lazarus when he was sick) only to lead us into a state of being better for us than where we were.
The disciples had been given authority over the demons, but it seems they did not believe, and thus the demon was not cast out. Later when they were clothed with power on Pentecost even St. Peter's shadow was effectual for help. We are granted to have communion with Christ and to call on Him and all of heaven to come to our aid. With all the variety of ways that Christ can come to our aid, and the variety of ways we might respond to any delay or unexpectedness, one thing is for certain: Jesus Christ is the same yesterday, today, and always. He is faithful, even if we are not. He has power over all things, and all things are under His feet - physical and spiritual. As St. Paul says in Ephesians 1:15-23,
Therefore I also, after I heard of your faith in the Lord Jesus and your love for all the saints, do not cease to give thanks for you, making mention of you in my prayers: that the God of our Lord Jesus Christ, the Father of glory, may give to you the spirit of wisdom and revelation in the knowledge of Him, the eyes of your understanding being enlightened; that you may know what is the hope of His calling, what are the riches of the glory of His inheritance in the saints, and what is the exceeding greatness of His power toward us who believe, according to the working of His mighty power which He worked in Christ when He raised Him from the dead and seated Him at His right hand in the heavenly places, far above all principality and power and might and dominion, and every name that is named, not only in this age but also in that which is to come.
And He put all things under His feet, and gave Him to be head over all things to the church, which is His body, the fullness of Him who fills all in all.