Mitochondrial diseases do exist, but (my understanding is that) they're pretty rare and usually genetic. And, I would expect that something like that would have been diagnosed pretty early in a person's life, since the mitochondria do quite a bit for the body.
The only time I've ever heard of a real-life case was second-hand, a friend of a friend, who I never met, but apparently she was always very weak and easily tired, but not on the verge of death at any point. Of course, that's just one person with one kind of mitochondrial disease.
Also, ditto on the hospital and parents issue.