William has a disease that's very rare: Epidermolysis. Known as an incredibly rare birth defect, epidermolysis (also known as the Butterfly Disease) causes the skin to basically slough off when disturbed. Because of the skin’s fragility, it’s common for sufferers to be covered in painful blisters and open wounds. These wounds frequently become infected, filling with pus and mucus. The skin is so sensitive that even something as simple as a temperature change can cause debilitating damage.