Sickness is an oppression of the devil. God is a creator not a destroyer. The two are opposites and cannot mix. Some Christians look at other people's experiences and doubt, instead of looking at the promises of God. It is Satan who makes people doubt.
Faith makes people believe.
A seed will always produce that which is in it. But it cannot do so if it falls on barren land. God sent Jesus to destroy the works of the devil. Jesus came to set the sick free from oppression by the devil. Jesus is the same today.