Why did Mary Ingalls go blind?

Are there any Little House on the Prairie fans here? I absolutely loved the books and the tv show when I was younger. I wanted to be Laura Ingalls more than anything! Remember how we all thought her sister Mary went blind due to Scarlett fever? That wasn't it. Her case was studied and it turns out she most likely went blind due to other reasons. Check this story out: Mary Ingalls Blindness