Two Steams

The principle is that healing for unbelievers has a different basis from healing for believers. Unbelievers are in the kingdom of darkness, so it is normal for them to be sick. They can only be healed if God breaks in with healing power and mercy that they do not deserve. On the other hand, Christians are living in Christ, so they should be experiencing all the benefits of the cross. They are entitled to the healing earned when strips of flesh were ripped off Jesus back. Therefore, it as abnormal for Christians to be sick. If Christians are sick something serious is wrong in the body of Christ.

Unbelievers need the gospel to be freed from the kingdom of darkness. God will sometime extend mercy to an unbeliever to prove his existence and mercy and love. He does not do this just for the benefit of the person healed, but for all those who see the miracle. He releases his power to shake people up and make them ready to receive the gospel.

Christians get sick because they have not received something that they are entitled too. The solution is not physical power, but wisdom to sort out the obstacle that is preventing them from receiving what Jesus has earned for them. If they cannot get that wisdom on their own, they will need help from someone else with greater faith or wisdom. This is why scriptures tell sick Christians to go to their elders. There may be a variety of reasons they Christian is sick: lack of faith, sin, a demon, ignorance of the gospel or spiritual strongholds, bush fever. The elders will seek God for the reason. When the elders know what it is, they can deal with it and allow the sick person to receive the health that Jesus has bought from them.

The cause of sickness is different for unbelievers and believers, so of course the solution must be different.