"By the Shores of Silver Lake"
This book opens just after the Ingalls family has been hit with an outbreak of scarlet fever. Ma, Mary, Carrie and the newest child, Grace, all came down with the illness. This is when Mary went blind.
“She was able to sit up now, wrapped in quilts in Ma’s old hickory rocking chair. All that long time, week after week, when she could still see a little, but less every day, she had never cried. Now she could not see even the brightest light any ore. She was still patient and brave.” [By the Shores of Silver Lake, page 2]
From this point on, Laura becomes Mary’s eyes, describing scenes and objects for her older sister.
“Laura tried to tell her how fast the telegraph poles were going by. She said, ‘The wire sags down between them and swoops up again,’ and she counted them. ‘One--oop! two--oop! three! That’s how fast they’re going.’ “ [By the Shores of Silver Lake, page 22] 
This quote is especially significant--because Laura had to become Mary’s “speaking eyes,” so to speak, learning how to describe what she was seeing to Mary taught her how to use words to paint a picture and how to “show, not tell.”
Because members of the Ingalls family became ill with scarlet fever, these quotes underscore that, while time was moving and the girls were growing, they were still vulnerable to potentially dangerous illnesses.