Who would think that I, of all people, would seed this? I read constantly, and have since I was a little kid. So I remember the Little House on the Prairie books an wish I still had my originals.
I seem to recall it was "fever and ague" which I took as malaria. I don't remember seeing the term "scarlet fever."
A mystery solved! As hard as it would be to be blind now, imagine that happening when you were 14 years old, living on a prairie ... Mary was one of the lucky ones.