Romans 8:29:

For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren.

You are supposed to resemble Jesus Christ on this earth. He is your elder brother, and you should bear a family resemblance! When people see you, they should see Jesus.

Jesus Christ was not sick until he took your sins. He walked the earth in divine health. You are supposed to be conformed to his image. Therefore, you should walk in divine health, too.

No one who looks at someone suffering some terrible illness will see an accurate image of Jesus Christ on the earth. Jesus Christ walked in authority over sickness and gave his disciples the same authority. Being well is part of being conformed to Christ's image.