Board Thread:WARNING ALL USERS False Info\Spoilers/@comment-24534214-20140211203920/@comment-13965604-20140211211536

Frontotemporal dementia is hereditary. The Nogitsune is either deliberately mimicking the symptoms to increase pain and suffering to feed on, or Claudia was also actually being possessed and it led to her death.

However, no one has said that Stiles got the disease from sacrificing himself. Though it would make sense that depriving his brain of oxygen would jump start a degenerative brain disease such as dementia. As I said, it's a hereditary disease.