In this case, Jesus said that his blindness was
not because of the man's sin or his parent's,
"but that the works of God should be made
manifest in him."

Even though much sickness and disease is a
result of generational sin and individual sin
opening a way for the devil; there is sin in the
world.

But Jesus said this was that the works of God
should be made manifest in him. It is not good
to blame every affliction on people to be of their
own doing or of their ascendants (inherited).

In John 5:1-14 the man at the pool that Jesus
healed, Jesus did tell him "Behold, thou art
made whole: sin no more, lest a worse thing
come unto thee."