Why is Daenerys immune to fire but not Viserys?

Why is Daenerys immune to fire but not Viserys? In the books, Daenerys is not immune to fire. The event in which the dragon eggs hatched was magical, according to George R.R. Martin. This protected Daenerys to a degree, but she still lost all of her hair and suffered fairly bad burns on her hands and feet.

In the books, Daenerys is not immune to fire. The event in which the dragon eggs hatched was magical, according to George R.R. Martin. This protected Daenerys to a degree, but she still lost all of her hair and suffered fairly bad burns on her hands and feet.