Mark 9:14 - 29
AMP - 14 And when they came to the [nine] disciples, they saw a great crowd around them and scribes questioning and disputing with them. 15 And immediately all the crowd, when they saw Jesus [returning from the holy mount, His face and person yet glistening], they were greatly amazed and ran up to Him [and] greeted Him.
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All the above answers have been excellent. May I add another possibility that takes more literal approach? There are forms of epilepsy (recurring seizures) that can be controlled by fasting- short term starvation that puts the body into a state of ketosis. The brain requires glucose for energy. It can be forced to use ketones instead of glucose for functioning during this starvation condition. This transformation can cause the cessation of seizing. However, being in ketosis may not cure the underlying problem, thus needing God's healing powers for a total permanent cure. Thus the reason for Christ's statement of this type needing prayer as well as fasting! I just love how God's word can be taken literally and all of the spiritual implications that can be implied. Hope this helps someone and thankful to be able to contribute. BTW, you can look up Ketogenic Diet for more information. It saved my daughters live, along with many peoples prayers!