I'd read that J.K. Rowling's mother died of multiple sclerosis, but it didn't occur to me how strongly it influenced her books:

"I know I was writing Harry Potter at the moment my mother died," she says. "I had never told her about [him].

"Barely a day goes by when I do not think of her. There would be so much to tell her, impossibly much."
…

The death left her "a wreck", she says, and was the inspiration for Harry's orphan status. (Source)