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By the Shores of Silver Lake, one of the beloved Little House on the Prairie novels, begins with the following account:

Mary and Carrie and baby Grace and Ma had all had scarlet fever. The Nelsons across the creek had had it too, so there had been no one to help Pa and Laura. The doctor had come every day; Pa did not know how he could pay the bill. Far worst of all, the fever had settled in Mary's eyes, and Mary was blind.

For many young readers, the account struck fever in the hearts. The lives of Mary, who was the well-behaved sisterly nemesis to main character Laura, and of the readers were inexplicably changed. However, researchers have long wondered about the mystery of Mary Ingalls, now believing that many cases of blindness then attributed to scarlet fever were not caused by scarlet fever at all. So what caused Mary Ingalls's blindness?

A study conducted by researchers from the University of Michigan's C.S. Mott Children's Hospital purport to know the answer. By combing contemporary documents, including Laura Ingalls Wilder's own letters, they found evidence that 14-year-old Mary had suffered from symptoms of a stroke. A local newspaper wrote off a hemorrhage that Mary had suffered, causing partial paralysis in her face. According to Health Day, Wilder herself wrote in a 1937 letter to her daughter that her sister had suffered from "spinal meningitis", before crossing out those words and replacing them with "some sort of spinal sickness".

The researchers believe that Mary did not suffer from scarlet fever, but rather viral meningoencephalitis. That "could explain Mary's symptoms, including the inflammation of the facial nerve that left the side of her face temporarily paralyzed," study senior author Beth Tarini said in a statement, "and it could also lead to inflammation of the optic nerve that would result in a slow and progressive loss of sight."

However, not everyone is convinced by this rewriting of the mystery. Dr. Bruce Hirsch, from North Shore University Hospital, agreed that scarlet fever was not the cause of Mary's blindness. However, he believes that Ingalls likely suffered from a viral infection and a fever that caused dehydration, which blocked a vein that supplies the eyes with blood. "If meningoencephalitis caused enough nerve damage to blind you, it would be unusual for it to just hit that part of the brain without causing a more general injury," he argued to HealthDay.

Researchers believe the real cause of Mary's blindness was changed by the editors, because scarlet fever was rampant in the late 19^th century and easier for readers to understand.

Scarlet fever is treatable today, but back then scarlet fever was fatal in 15 to 30 percent of cases, CBS reports.

The study was published in the journal Pediatrics.