Humans depend on God's mercy. So if one preaches about the healing power of faith, nothing should be said that sounds like the preacher already knows God's will. A sick person should never be told to discontinue medical treatment. Prayer for healing should accompany the taking of medication, not replace it. Illness and possession by evil spirits should not be confused.
Most illnesses are just illnesses, and have nothing to do with evil spirits.
Sick people should therefore not normally undergo an exorcism. The sick person should not just rely on the prayers of other people. While all Christians should include their sick brothers and sisters in their prayers, the sick person's direct contact with God is most important.