In the Bible, God has a precedent for giving people afflictions. He gave Job lots of afflictions. He gave the people of Egypt lice. At one point he gave the entire world the affliction of death by drowning in a flood.
Did God create everything? Does He know everything that's going to happen? Is He all-powerful? If the answer is "yes", then He is responsible for the disease. It is a genetic disease from a design that He created knowing full well that this is the destiny that awaits this child. It is from an environment that He created and controls. It is in His power to cure this disease and yet the most effective cure is not prayer but medical science.
Was going to point that out but you beat me to it!
Old Testament truly has some messed up shit.