Many people may ask: when there is no need for believers to be sick, why does God not remove the sickness? We have to realize that God accomplishes things according to our faith (Matt. 8:13). This is an unchanging principle by which God deals with us. Many times, God is willing to heal His children. But since they do not believe and do not pray, God has to allow the sickness to remain. If a believer allows himself to be sick, or worse still, welcomes sickness, thinking that it will make him more unworldly and holy, the Lord can do nothing except give him what he wishes. God often deals with His children according to what they can take. God may want very much to heal them, but because they do not have the faith to ask, they never receive this gift.
Read here: https://clifestream.com/sickness/