For some, Down Syndrome can be deadly.
There is a blog out there, Lives Lost To Autism, with the tag-line, "For many, autism can be deadly." It is authored by a variety of people, including Kim Stagliano, an excellent autism advocate for the whole ASD spectrum and a parent of three girls with autism.
I do not think ASD is - by itself, fatal. However, lack of awareness of danger coupled with improper or no supports can put a child or adult with ASD in a very dangerous and possibly life-threatening situation (i.e. wandering and getting lost).
ASD is not the only medical condition that can put a person in a life-threatening situation.
Down Syndrome, for some people, can cause serious and terminal illnesses that could lead to death.
Scott Widak, 47, died of liver disease.
Kristin Kirton, 22, died of Leukemia (ALL).
Martin Ryan, 43, died of 26-day starvation.