Or as The Message puts it: You're asking the wrong question. You're looking for someone to blame. There is no cause and effect here. Look instead for what God can do.
There's the text for World Bipolar Day.
In the Gospel, Jesus heals a man born blind. Presumably what God can do is made manifest by that healing. So, okay, Jesus, what about me?
What about me? How many people, with how many disabilities, wonder what God is doing, especially those of us surrounded by others who wonder, Who sinned, this one or the parents?