Is sickness always due to sin?

Certainly Job's sickness wasn't! Yet his friends did insist that if he would only stop his wickedness, all his fortunes would be restored, but God said they did not speak what was right about him.
 
And Lazarus was sick, and Mary and Martha asked Jesus to come and heal him, and Jesus waited, and Lazarus, his friend Lazarus, then died. Some healings are resurrection healings...

"God is presented not only as one who sends affliction as a teaching or punitive device, but also as one who can use affliction to further the spread of the Gospel. Two NT texts present God as using infirmity in precisely this manner. Both the blind man in John 9 and Paul, who suffers from a weakness in the flesh in Galatians 4, experience infirmity in order that God might accomplish his purpose through the revelation of his message. With the blind man the reader is told that this condition exists in order that the works of God might be revealed. Such a statement is a response to the disciples' question about the origin of the man's condition and at the same time a sign that such an action on God's behalf is not viewed as something distasteful by John. Clearly this statement conveys the sense that God may send (or use) affliction to suit his purpose, in this case the manifestation of his works, in order to generate the faith that leads to eternal life. In similar fashion, Paul's illness described in Galatians (4) results in the preaching of the Gospel to the Galatians. This illness, which could have proven to be a stumbling block or obstacle to the Galatians, turns out to be the very occasion for them to hear the message of salvation about Jesus Christ. The implication is that God's hand can be seen even in this illness, for it serves his ultimate purpose." (John Christopher Thomas)