Laura Ingalls and her sister Mary—whose lives were immortalized on Little House on the Prairie, and in the Little House series of books on which the TV show was based—were real people who were really pioneers in the 19th century. And the real Mary indeed went blind in 1879 when she was 14. Both the books and TV show credit her loss of eyesight to a bout of scarlet fever. But a new article in the journal of the American Academy of Pediatrics explains that it couldn't have been from scarlet fever—because scarlet fever doesn't cause blindness.
In the fourth book in the series, By the Shores of Silver Lake, Laura writes:
Mary and Carrie and baby Grace and Ma all had scarlet fever…Far worst of all, the fever had settled in Mary's eyes and Mary was blind.
On the TV show, Mary's vision begins to decline, necessitating glasses. Eventually, she wakes up one morning screaming that she couldn't see at all. A doctor told Pa that scarlet fever had "weakened the nerves" in her eyes. But Dr. Beth Tarini, assistant professor of pediatrics at the University of Michigan, and co-author of "Blindness in Walnut Grove: How Did Mary Ingalls Lose Her Sight" says that "[C]linically, it doesn't make sense." However, it was a widely held belief:
As late as 1910, scarlet fever was cited as one of the top four causes of blindness, along with measles, meningitis, and "other diseases of the head."
For her research, Dr. Tarini enlisted the help of some other doctors, an Ingalls family historian, local newspapers from the time, and the original draft of Laura's unpublished memoir Pioneer Girl to find out what truly caused Mary's blindness.
In 1937, in a letter to her daughter Rose, Laura wrote:
Mary had spinal meningitis [sic] some sort of spinal sickness. I am not sure if the Dr. named it. We learned later when Pa took her from De Smet, South Dakota to Chicago, Illinois to a specialist that the nerves of her eyes were paralyzed and there was no hope.
In Pioneer Girl, Laura described Mary as suffering from "a severe case of measles" that caused a stroke that caused lifelong blindness. But the register at Mary's school, Iowa College for the Blind, lists the cause as "brain fever." So what the hell really happened to Mary?
After reviewing the local papers at that time and Laura's descriptions of her sister's illness, researchers concluded that Mary had contracted viral meningoencephalitis, an inflammatory disease that attacks the brain, colloquially referred to as "brain fever." So it would seem that Mary had the right diagnosis, according to her college records. So why did Laura change the illness to scarlet fever?
Well, Laura took a lot of liberties with the truth when it came to her books, which are considered "historical fiction" instead of "memoir," (her age is incorrect in the books and there were many revisions because the original manuscript "lacked drama") and there are even more discrepancies in the show. (Mary never married her teacher from her blind college, nor did she open her own school for the blind. Instead, she lived with her parents until their deaths and made a living making fly nets for horses.) Additionally, scarlet fever was an already tried and true literary device at the time, featured prominently in Little Women, Frankenstein, and the Velveteen Rabbit. Perhaps Laura's editors felt that the illness would be better understood by children, instead of the multi-syllabic viral meningoencephalitis.