Is Sickness the Result of Sin?
by John Piper, quoted from his article, "Satan's Ten Strategies Against You"
Don’t make the mistake of saying every sickness is the work of the devil. To be sure, even when a “thorn in the flesh” is God’s design for our sanctification, it also may be the “messenger of Satan” (2 Corinthians 12:7). But there are other instances in which the disease is solely attributed to God’s design without reference to Satan: “It was not that this man sinned, or his parents, but that the works of God might be displayed in him” (John 9:3). Jesus feels no need to bring Satan in as the culprit in his own merciful designs.See quote source