My aunt became very ill and eventually died of the illness. She was a devote Christian and had great faith that God would heal her. She believed she would be healed through faith and prayer. She continually asked God to heal her. However, she finally came to the realization that God said NO. For what reason no one knowns, but NO it was. That was the day she died. She let God take her instead of fighting Him on the issue.

God can  heal anyone He decides to heal. [either immediately or in the long run of his timing]  God, however,  is not at our beck and call  no matter what level of faith we have.   We can ask, present our case,  and accept his answer. God may have  a reason for His NO answer and it might be better for us that He did not grant our request.  We cannot demand anything of God.  We are to do His Will and not He to  do our will.