Scarlet fever is treatable with antibiotics, but in the middle of the 19th century, it was the leading cause of death in children in some parts of the world. Today, there are several ongoing mysteries about the disease.
Research:

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Scamman, Clarence L. “Milk-Borne Septic Sore Throat and Scarlet Fever.” American Journal of Public Health. December 1929. https://www.ncbi.nlm.nih.gov/pmc/