How rare is sporadic fatal insomnia?

SFI. Only 8 cases have ever been diagnosed as of july 2005, so it is extremely rare.
Yes, EXTREMELY rare. I read that 16 cases of sporadic fatal insomnia are in the literature as of 2011 . . . . Which still makes it extremely rare! http://www.Biomedcentral.Com/1471-2377/11/136.